3.8: The Thyroid Gland - Biology

3.8: The Thyroid Gland - Biology

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Learning Objectives

By the end of this section, you will be able to:

  • Describe the location and anatomy of the thyroid gland
  • Discuss the synthesis of triiodothyronine and thyroxine
  • Explain the role of thyroid hormones in the regulation of basal metabolism
  • Identify the hormone produced by the parafollicular cells of the thyroid

A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx (Figure 1). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones’ essential and unique component: iodine.

Synthesis and Release of Thyroid Hormones

Hormones are produced in the colloid when atoms of the mineral iodine attach to a glycoprotein, called thyroglobulin, that is secreted into the colloid by the follicle cells. The following steps outline the hormones’ assembly:

  1. Binding of TSH to its receptors in the follicle cells of the thyroid gland causes the cells to actively transport iodide ions (I) across their cell membrane, from the bloodstream into the cytosol. As a result, the concentration of iodide ions “trapped” in the follicular cells is many times higher than the concentration in the bloodstream.
  2. Iodide ions then move to the lumen of the follicle cells that border the colloid. There, the ions undergo oxidation (their negatively charged electrons are removed). The oxidation of two iodide ions (2 I) results in iodine (I2), which passes through the follicle cell membrane into the colloid.
  3. In the colloid, peroxidase enzymes link the iodine to the tyrosine amino acids in thyroglobulin to produce two intermediaries: a tyrosine attached to one iodine and a tyrosine attached to two iodines. When one of each of these intermediaries is linked by covalent bonds, the resulting compound is triiodothyronine (T3), a thyroid hormone with three iodines. Much more commonly, two copies of the second intermediary bond, forming tetraiodothyronine, also known as thyroxine (T4), a thyroid hormone with four iodines.

These hormones remain in the colloid center of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and T4, which diffuse across the follicle cell membrane and enter the bloodstream.

In the bloodstream, less than one percent of the circulating T3 and T4 remains unbound. This free T3 and T4 can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, bound T3 and T4 are released from these plasma proteins and readily cross the membrane of target cells. T3 is more potent than T4, and many cells convert T4 to T3 through the removal of an iodine atom.

Regulation of TH Synthesis

The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in Figure 2, low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH.

Functions of Thyroid Hormones

The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.

Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.

Disorders of the Endocrine System

Iodine Deficiency, Hypothyroidism, and Hyperthyroidism

As discussed above, dietary iodine is required for the synthesis of T3 and T4. But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today.

Dietary iodine deficiency can result in the impaired ability to synthesize T3 and T4, leading to a variety of severe disorders. When T3 and T4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter (Figure 3). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable mental retardation worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.

In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity.

In contrast, hyperthyroidism—an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.


The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:

  • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
  • Increasing osteoblastic activity
  • Decreasing calcium absorption in the intestines
  • Increasing calcium loss in the urine

However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in Table 1.

Table 1. Thyroid Hormones
Associated hormonesChemical classEffect
Thyroxine (T4), triiodothyronine (T3)AmineStimulate basal metabolic rate
CalcitoninPeptideReduces blood Ca2+ levels

Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands.

Chapter Review

The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid–derived T3 and T4 hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

  1. Explain why maternal iodine deficiency might lead to neurological impairment in the fetus.
  2. Define hyperthyroidism and explain why one of its symptoms is weight loss.

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  1. Iodine deficiency in a pregnant woman would also deprive the fetus. Iodine is required for the synthesis of thyroid hormones, which contribute to fetal growth and development, including maturation of the nervous system. Insufficient amounts would impair these functions.
  2. Hyperthyroidism is an abnormally elevated blood level of thyroid hormones due to an overproduction of T3 and T4. An individual with hyperthyroidism is likely to lose weight because one of the primary roles of thyroid hormones is to increase the body’s basal metabolic rate, increasing the breakdown of nutrients and the production of ATP.



calcitonin: peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels

colloid: viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin

goiter: enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism

hyperthyroidism: clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate

hypothyroidism: clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity

neonatal hypothyroidism: condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine-deficient during pregnancy

thyroid gland: large endocrine gland responsible for the synthesis of thyroid hormones

thyroxine: (also, tetraiodothyronine, T4) amino acid–derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells

triiodothyronine: (also, T3) amino acid–derived thyroid hormone that is less abundant but more potent than T4

The Thyroid Gland

The T3 receptor is a nuclear protein bound to the thyroid response element in the promoters of the many genes whose expression is influenced by thyroid hormones. When its ligand, T3, binds to it, it becomes a transcription factor turning on the transcription of many genes.

2. hyperthyroid diseases caused by excessive secretion of thyroid hormones

Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.

Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.

Mcq in biology thyroid gland

1) the thyroid gland is originated from which part of the embryo

A) ectoderm
B) mesoderm
C) endoderm
D) none

2) the thyroid gland is which of the following

A) trilobed
B) bilobed
C) tetralobed
D) hexalobed

3) the two lobes are connected by a narrow band is called as which of the following

A) isthmus
B) follicles
C) capsule
D) colloid

4) stroma of thyroid gland is enclosed by which part

A) isthmus
B) follicles
C) capsule
D) colloid

5) stroma of thyroid gland having which type of cells

A) isthmus
B) follicles
C) capsule
D) para follicular or C cells

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6) which hormone are secreted by thyroid glands

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) all of these

7) which of the following is thyroxine hormones

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) all of these

8) which hormone stimulates the thyroid gland to Secrets thyroxine hormone

A) thyrotrophin
B) calcitonine
C) prolactine
D) gonadotropin

9) thyroitrophine hormone released by which gland

A) thyroid gland
B) hypothalamus
C) anterior pituitary glands
D) posterior pituitary glands

10) which one of the following hormone is iodinated forms of amino acid tyrosine

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) both A and B

11) which hormone maintain the basal metabolic rate of our body

A) thyroxine
B) thyrotrophin
C) gonadotrophin
D) somatotrophin

12) which hormone affect the metamorphosis of tadpole into a adult frog

A) thyroxine
B) thyrotrophin
C) gonadotrophin
D) somatotrophin

13) which hormone promote growth of body tissue and development of mental faculty

A) thyroxine
B) thyrotrophin
C) gonadotrophin
D) somatotrophin

14) which hormones control the working of Kidneys

A) thyroxine
B) thyrotrophin
C) gonadotrophin
D) somatotrophin

thyroid and parathyroid glands MCQs with answers for neet-CBSE-JAC exams

15) Grave’s disease exophthalmic goitre is caused due to

A) hyposecretion of thyroxine hormone
B) hypersecretion of thyroxine hormone
C) both
D) none

16) excessive leanness is caused due to

A) hyposecretion of thyroxine hormone
B) hypersecretion of thyroxine hormone
C) both
D) none

17) what is the main cause for the increase of basal metabolic rate

A) hyposecretion of thyroxine hormone
B) hypersecretion of thyroxine hormone
C) both
D) none

18) exccessive thyroid hormone is released due to

A) enlargement of thyroid glands
B) contraction of thyroid glands
C) removal of thyroid gland
D) none

19) what is the main cause of cretinism in infant

A) hyposecretion of thyroxine hormone
B) hypersecretion of thyroxine hormone
C) both
D) none

20) slower basal metabolic rate is caused due to

A) hyposecretion of thyroxine hormone
B) hypersecretion of thyroxine hormone
C) both
D) none

21) Mxyoedema is caused by

A) deficiency of thyroid hormone in adults
B) deficiency of thyroid hormone in infants
C) deficiency of thyroid hormones in male only
D) over secretion of thyroid hormones

22) Cretinism is caused by

A) deficiency of thyroid hormone in adults
B) deficiency of thyroid hormone in infants
C) deficiency of thyroid hormones in male only
D) over secretion of thyroid hormones

23) iodine deficiency goitre is caused by

A) deficiency of iodine in the diet
B) over secretion of thyroid hormone
C) deficiency of thyroid hormone
D) none

24) which element it is required for the synthesis of thyroid hormones

A) iron
B) copper
C) iodine
D) bromine

25) overactivity of thyroid is known as which of following

A) hyperthyroidism
B) hypothyroidism
C) both
D) none

26) underactivity of thyroid is known as which of following

A) hyperthyroidism
B) hypothyroidism
C) both
D) none

27) which hormone is secreted by the C cells

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) all of these

28) what is the main function of calcitonin

A) it regulates the concentration of calcium and phosphorus in the blood
B) it regulate the concentration of iodine and phosphorus in blood
C) it regulate the concentration of iron and phosphorus in blood
D) it regulate the concentration of Copper and phosphorus in blood

29) what is the main cause of release of calcitonin in blood

A) it is secreted when calcium concentration rises in blood
B) it is secreted when calcium concentration decrease in blood
C) it is secreted when iron concentration increase in blood
D) it is secreted when iron concentration decrease in blood

30) the parathyroid glands is arises from which part of embryo

A) ectoderm
B) mesoderm
C) endoderm
D) none

31) where is parathyroid gland situated

A) in chest region
B) in abdominal cavity
C) in brain
D) posterior surface of the thyroid lobes

32) what is the number of parathyroid glands

A) 4 in number
B) 3 in number
C) 2 in number
D) 1 in number

33) what are the cells of parathyroid glands arrange in a compact mass

A) chief cells
B) oxyphils cells
C) C cells
D) both A and B

34) smaller cell present in parathyroid glands is

A) chief cells
B) oxyphils cells
C) C cells
D) both A and B

35) larger cell present in parathyroid Gland is

A) chief cells
B) oxyphils cells
C) C cells
D) both A and B

36) which hormone is secreted by parathyroid gland

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) parathormone

37) which hormone is known as Collip’s hormone

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) parathormone

38) which hormone is is function as opposite to calcitonin

A) Tetraiodothyronine ( T4)
B) Triiodothyronine ( T3)
C) calcitonine
D) parathormone

39) hyposecretion of parathormones hormones causes

A) hyperthyroidism
B) hypothyroidism
C) hypoparathyroidism
D) hyperparathyroidism

40) hypersecretion of parathormones hormones causes

A) hyperthyroidism
B) hypothyroidism
C) hypoparathyroidism
D) hyperparathyroidism

The Link Between Your Thyroid And Adrenal Glands

Thyroid problems are one of the most common conditions we treat at our clinics. They are especially common in women. They are a big cause of fatigue and inability to lose weight.

Thyroid problems can develop at any age, but they are especially common in women who have recently given birth and women who have recently gone through menopause. The hormonal shifts at this time are a trigger for abnormal thyroid function.

Hashimoto’s thyroiditis is the most common cause of an under active thyroid gland. The usual treatment is prescription thyroid hormone – either synthetic hormones known as levothyroxine (eg. Synthroid), or natural porcine thyroid hormones (commonly referred to as Armor).

The problem is, many people with a thyroid condition are already taking one of these medications but they continue to suffer with symptoms of an under active thyroid gland. That is, they are still tired, depressed, suffer with fluid retention and can’t lose weight. What causes these ongoing symptoms? Sometimes the answer is adrenal gland dysfunction. Fixing the adrenal glands is sometimes the key to overcoming thyroid symptoms.

Your thyroid gland is a butterfly shaped organ at the front of your neck. It performs many essential functions related to growth, metabolism, body temperature, hormonal control (including menstrual cycle regulation), and mood. Your adrenal glands are two tiny triangular glands sitting on top of your kidneys. They control your stress response, immune function, blood pressure, blood sugar and your reproductive hormones.

If you experience prolonged chronic stress, your adrenal glands tell your body to conserve energy. Your metabolism slows down, you feel tired and want to sleep more and may experience more food cravings. Your thyroid gland function slows down, too. It’s like a warning system that tries to force your body to get more rest and sleep. Chronically over worked adrenal glands have a major negative impact on your immunity – over time this increases your risk of developing chronic inflammation and autoimmunity. These factors can lead to an eventual development of Hashimoto’s thyroiditis, which causes autoimmune hypothyroidism and is responsible for around 90 percent of all thyroid dysfunction in the USA.

When you are experiencing chronic stress, your immune cells release chemicals called inflammatory cytokines. Some examples include IL-1 beta, IL-6 and TNF-alpha. These chemicals have the unfortunate side effect of down regulating the production of the thyroid hormones TSH, T3, and T4. They make your thyroid less sensitive to TSH and decrease the conversion of the inactive form of the thyroid hormone T4 to the active form, T3. Chronic inflammation can make the thyroid hormone receptors on your cells less sensitive to active thyroid hormones. This means your thyroid hormones can’t do their job properly. This is called thyroid hormone resistance.

Reducing stress and improving the health of your adrenal glands can help improve thyroid health

Here are my tips

  • Try to get adequate sleep. People with adrenal fatigue often have poor quality sleep, but your adrenal glands will greatly benefit if you can be in bed before 10:30pm. The hours before midnight are much more restorative to your body. Magnesium is a great tool to promote a deeper and more restful sleep.
  • If you have low levels of cortisol (typical with adrenal gland exhaustion, and can be measured in a blood test), you may benefit from taking Adrenal Natural glandular capsules. They contain natural glandular material derived from government-inspected, range-fed animals, raised in New Zealand and Australia. This provides your body with natural adrenal gland hormones that help to relieve the symptoms of exhaustion and low stamina. It is important to take this in the morning.
  • Try to reduce the stress in your life or find more effective ways of dealing with it. Massage, meditation, yoga, dancing classes, facials, counseling, reading or going for a bike ride can all be healthy ways to unwind.
  • Many people with a thyroid condition are low in the nutrients required for the production of thyroid hormones. Being low in these nutrients can result in fatigue, low mood and impaired concentration. These nutrients are found in Thyroid Health capsules.
  • Make sure you eat enough protein and healthy fats. These two nutrients will help to keep your blood sugar stable throughout the day, thereby reducing energy slumps caused by a blood sugar crash.
  • You may benefit from a tyrosine supplement. Tyrosine is an amino acid that your body uses to manufacture adrenalin and noradrenalin. It helps you to emotionally cope with stress more effectively and even helps mood and concentration.
  • Most of the vitamin C in your body is stored in your adrenal glands. Most people do not consume enough fresh vegetables and fruit to get sufficient vitamin C in their diet. Your adrenal glands will struggle to function unless you consume enough vitamin C. If you have adrenal exhaustion you will need at least 1000mg of vitamin C daily.

The above statements have not been evaluated by the FDA and are not intended to diagnose, treat or cure any disease.

What Does It Mean When Your Thyroid Gland Is Heterogeneous?

A heterogeneous thyroid gland means that the gland is abnormal, according to the Research Institute of Radiological Science. A normal, or homogeneous, thyroid has uniform tissue throughout. An abnormal thyroid gland often presents with nodules, which may or may not be cancerous.

Most thyroid nodules are benign, according to both the Research Institute of Radiological Science and Mayo Clinic. Some are even silent. However, they still indicate possible thyroid disease and need to be investigated by a medical professional. Even a cancerous thyroid has a good prognosis if it's detected early.

If thyroid nodules cause symptoms, they might present as lumps that are felt or seen in the neck. Some are large enough to interfere with swallowing or breathing, states Mayo Clinic. Some stimulate the thyroid to produce excess thyroid hormones, which leads to hyperthyroidism symptoms.

These symptoms include weight loss, even though the patient isn't trying to lose weight, or an increased appetite, tachycardia, trembling, weakness, nervousness and an inability to tolerate heat, claims Mayo Clinic. Complications of hyperthyroidism can be osteoporosis, or weakened bones, and a thyrotoxic crisis. This is a potentially fatal development that requires immediate medical attention.

A heterogeneous thyroid can be caused by an iodine deficiency or goiter, cysts, inflammation, overgrowth of thyroid tissue and cancer, also according to Mayo Clinic.

The Thyroid Gland

▶Anatomy of the Thyroid Gland
The thyroid gland lies near thyroid cartilage of the larynx and is known to be a butterfly in shape. It consists of two lobes connected by the isthmus. It has a rich blood supply. Also, it contains numerous thyroid follicles. It produces, store and releases thyroid hormones.

▶Thyroid Follicles and Thyroid Hormones
Thyroid Follicles produce and store colloid. Thyroid colloid consists of enzyme required for secretion of thyroid hormone. The production of colloid requires adequate iodine in the diet. The thyroid follicles lead to the production of thyroxine (T4) and triiodothyronine (T3). It increases metabolism and heat production (calorigenic effect) ie. it results in increased uptake of oxygen which increases the respiration rate and ultimately in ATP production.

▶C Cells of the Thyroid Gland
C cells are scattered outside of follicle epithelium known as parafollicular cells. It secretes calcitonin and lowers the blood Ca2+ levels. It also inhibits osteoclasts of bone. It gets triggered by the hypothalamus when there is low Ca2+ level in the body.

If there is a fall in Ca2+ levels in body, the thyroid gland is triggered. This leads to the production of Calcitonin in the thyroid gland which increases the release of calcium ions in the kidney and the calcium deposition in the bone which maintains homeostasis.

▶The Parathyroid Glands

Posterior Thyroid glands are embedded by four glands. The parathyroid gland is the chief cells that produce parathyroid hormone. Lower blood Ca2+ levels trigger the secretion of this hormone. PTH speeds bone breakdown by osteoclasts and increases dietary absorption• It is triggered by high Ca2+ levels in the body.

If there is an increase in Ca2+ levels in the body, the parathyroid gland is triggered. This leads to the release of the stored calcium from bone. This enhances the reabsorption of calcium in the kidney which balances the calcium in the body maintaining homeostasis.

What do different TSH levels mean?

The thyroid stimulating hormone (TSH) test measures the amount of TSH in the blood. The results convey how well the thyroid is functioning.

Doctors can use TSH test results to diagnose thyroid disorders, such as hypothyroidism and hyperthyroidism.

The pituitary gland produces TSH, which is a hormone that stimulates the thyroid gland.

The thyroid is a butterfly-shaped gland in the throat. It produces hormones that help regulate many bodily functions, such as metabolism, heart rate, and body temperature.

In this article, we describe the TSH test and results. We also discuss what high and low TSH levels indicate and available treatments.

Share on Pinterest A TSH test can help a doctor diagnose hypothyroidism and hyperthyroidism.

The normal range depends on a person’s age and whether they are pregnant.

The ranges tend to increase as a person gets older. Research has not shown a consistent difference in TSH levels between males and females.

However, according to the American Thyroid Association, doctors generally consider levels to be within a normal range if they are between 0.4 and 4.0 milliunits per liter (mU/l).

The table below provides estimates of TSH levels that are normal, low (indicating hyperthyroidism) and high (indicating hypothyroidism):

hyperthyroidismnormalmild hypothyroidismhypothyroidism

Most labs use these reference values.

However, there is some debate about these ranges — the author of a 2016 review has found that normal levels are more likely to fall between 0.5 and 2.5 milli-international units (mIU) per milliliter.

Females are more likely to experience thyroid dysfunction than males. The Office on Women’s Health report that 1 in 8 females experience thyroid problems at some point. This includes hyperthyroidism and hypothyroidism.

The risk of thyroid problems increases during pregnancy and around menopause.

Research has not shown a consistent difference in TSH levels between males and females.

A 2002 study reports higher TSH levels in females than in males, but a 2013 study reports that males have higher median TSH levels. It appears that any such difference is small, varies with age, and is unlikely to be clinically relevant.

In some people, thyroid conditions are linked with sexual dysfunction. This may affect more males than females. According to a 2019 study, 59–63% of males who have hypothyroidism also experience sexual dysfunction, compared with 22–46% of females with hypothyroidism.

Author information


Laboratory of Fundamental Research, Volgograd Scientific Center, Russian Academy of Medical Sciences and Volgograd Region Administration, Russia

S. A. Kalashnikova, L. B. Polyakova & V. V. Novochadov

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Corresponding author

Correspondence to S. A. Kalashnikova.

What causes thyroid nodules?

Nodules can be caused by a simple overgrowth of normal thyroid tissue, fluid-filled cysts, inflammation (thyroiditis) or a tumor (either benign or cancerous). Most nodules were surgically removed until the 1980s. In retrospect, this approach led to many unnecessary operations, since fewer than 10 percent of the removed nodules proved to be cancerous. Most removed nodules could have simply been observed or treated medically.

Chronic thyroiditis (Hashimoto's disease) is an inflammation of the thyroid gland that develops slowly. It frequently leads to a decreased function of the thyroid (hypothyroidism). Thyroiditis occurs when the body's immune system destroys the cells in the thyroid gland. Chronic thyroiditis is most common in women and people with a family history of thyroid disease.

  1. Term Paper on Introduction to the Thyroid Gland
  2. Term Paper on Parts of the Thyroid Gland
  3. Term Paper on Capsule of the Thyroid Gland
  4. Term Paper on Blood Supply of the Thyroid Gland
  5. Term Paper on Nerve Supply of the Thyroid Gland
  6. Term Paper on the Parathyroid Glands
  7. Term Paper on the Thymus Glands

Term Paper # 1. Introduction to the Thyroid Gland:

This is a large endocrine gland situated in front of the lower part of the neck and overlies 2 – 4th tracheal cartilages. Its functions are controlled by the thyroid- stimulating hormone from the anterior (adeno) hypophysis.

It develops from endodermal epithelial proli­feration of median thyroid diverticulum.

The “C” cells of the gland are developed from the fourth branchial pouch.

Lower part of front of neck. The gland is situated in front of C5, C6, C7 and T1 vertebrae.

Term Paper # 2. Parts of the Thyroid Gland:

The gland has a pair of lobes united by an isthmus in the median plane.

Each lateral lobe has approximate dimensions as follows:

Vertical length is 1.25 cm

Lateral Lobe of Thyroid:

(a) Apex or upper pole is pointed and extends superiorly up to the oblique line of thyroid cartilage.

(b) Lower pole is blunt and rounded, extends inferiorly up to 6th tracheal ring. Lower pole on left side is related to thoracic duct.

(a) Anterior border is sharp and related to anterior branch of superior thyroid artery.

(b) Posterior border is blunt and rounded and is related to posterior branch of superior thyroid artery and ascending branch of inferior thyroid artery which anastomose with each other.

It is also related to parathyroid glands.

Surfaces are antero lateral or superficial surface, postero lateral surface and medial surface.

2. Superficial fascia containing platysma and cutaneous nerves and blood vessels.

3. Investing layer of deep cervical fascia.

7. Sternocleido mastoid muscle.

Superiorly the lobe is limited by “insertion of sternothyroid muscle”.

Medial Relations of Thyroid Lobe:

1. Two Viscera:

2. Two Muscles:

(b) Inferior constrictor of pharynx.

3. Two Nerve:

(a) External laryngeal nerve (X)

(b) Recurrent laryngeal nerve (X).

4. Two Cartilages:

Postero Lateral Relations of Thyroid Lobe:

1. Carotid sheath and its contents:

2. Cervical sympathetic trunk with its middle ganglion.

It crosses 2nd, 3rd and 4th tracheal rings.

Upper border shows pyramidal lobe and related to anastomosis between superior thyroid arteries.

Lower border is related to inferior thyroid vein.

Levator glandulae thyroidea connects pyramidal lobe to hyoid bone.

Term Paper # 3. Capsule of the Thyroid Gland:

It is formed by pretracheal fascia and is weakest posteriorly.

Pretracheal fascia thickens posteriorly and is attached to cricoid cartilage. This thickened band is called suspensory ligament of Berry.

It is formed by condensation of fascia around the gland. A plexus of veins lies deep to the true capsule. Therefore during thyroidectomy the gland is removed with its capsule to prevent bleeding.

Term Paper # 4. Blood Supply of the Thyroid Gland:

Thyroid gland is very vascular and supplied by:

1. Superior Thyroid Artery:

It is the first branch of external carotid artery. It descends downwards and forwards, reaches the upper pole of the gland – it divides into anterior and posterior branches to supply the gland near the apex. It is closely related to external laryngeal nerve. Hence during thyroid surgery artery is ligated close to the gland to save the nerve.

It supplies upper 1/3 of lateral lobe and upper 1/2 of isthmus of gland.

Anterior branch anastomoses with the branch of opposite side along the upper border of isthmus of gland.

2. Inferior Thyroid Artery:

It is a branch of thyrocervical trunk of subclavian artery. It passes deep to carotid sheath and middle cervical sympathetic ganglion, in front of vertebral vessels and reaches the posterior surface of the gland. It is accompanied by recurrent laryngeal nerve near the gland.

It divides into 4 or 5 branches and supplies lower 2/3 of the lateral lobe and lower half of isthmus of the gland. One branch ascends upwards along posterior border and anastomoses with the posterior branch of superior thyroid artery at the junction of upper 1/3 with lower 2/3 of the posterior border.

During surgery on thyroid this artery is ligated away from the gland to save recurrent – laryngeal nerve.

3. Thyroidea Ima Artery:

It is present only in 3% of individuals and is a branch from arch of aorta or brachiocephalic trunk. It runs upwards in the midline of neck to the isthmus of the gland.

4. Accessory Arteries:

Oesophageal and tracheal arteries also supply thyroid gland.

Veins do not follow arteries.

1. Superior Thyroid Vein:

Drains upper part of thyroid lobe, crosses anterior to common carotid and drains into internal jugular vein.

2. Middle Thyroid Vein:

Also crosses anterior to common carotid artery to end into internal jugular vein. It drains the middle part of the thyroid lobe.

3. Inferior Thyroid Vein:

A pair of veins emerging from the lower border of isthmus, crosses anterior to trachea and terminates into either brachiocephalic veins or left brachiocephalic vein.

Sometimes a fourth thyroid vein emerges between middle and inferior thyroid veins and drains into internal jugular vein.

Lymphatic drainage into:

1. Pre-laryngeal lymph nodes

2. Pre-tracheal lymph nodes

3. Deep cervical lymph nodes

4. Recurrent chain of lymph nodes along recurrent laryngeal nerve (para tracheal group).

Term Paper # 5. Nerve Supply of the Thyroid Gland:

Post ganglionic fibres from the superior, middle and inferior cervical sympathetic ganglia. These fibres form plexus around the thyroid arteries.

Parasympathetic Supply:

Vagus and recurrent laryngeal nerves.

The secretary functions of the gland is controlled by the T.S.H. of the anterior pituitary.

Accessory Thyroid (Ectopic Thyroid Tissue):

1. Lingual thyroid- Presence of thyroid tissue around the foramen caecum of the tongue.

2. Suprahyoid ectopic thyroid

3. Infrahyoid actopic thyroid

Ectopic thyroid tissue is found along the course taken by the thyroglossal duct.

Secretions of the Gland:

1. Thyroxin (T4) or Tetraiodothyronine.

Required for normal growth and develop­ment of body and maintain B.M.R. of body. (Both psychic and somatic growth.)

3. Thyrocalcitonin – secreted by the para follicular cells (“C” cells) – helps in deposition of calcium on bones – calcium metabolism.

Term Paper # 6. Parathyroid Gland:

Two pairs – superior and inferior parathyroid glands.

It is a small endocrine gland.

Lies on the posterior border of the thyroid gland within the false capsule.

Superior parathyroid develops from endoderm of 4th pharyngeal pouch and is also called parathyroid IV.

Inferior parathyroid develops from endoderm of IIIrd pharyngeal pouch and is called parathyroid III.

It secretes hormones called para-thormone, which controls the calcium and phosphorus metabolism along with thyrocalcitonin.

It causes resorption of calcium from bones and makes them weak and thus liable to fracture.

Shape – Oval or lentiform (half pea shaped)

Types of Parathyroid Gland:

It is more constant in position and lies at the middle of the posterior border, here it is slightly prominent. This prominence is called as tubercle of Zuckerkandl. It is dorsal to recurrent laryngeal nerve.

More variable in position.

1. Within the thyroid capsule below the inferior thyroid artery and near the lower pole of thyroid lobe.

2. Behind and outside the thyroid capsule immediately above the inferior thyroid artery.

3. Within the substance of the thyroid lobe and ventral to recurrent laryngeal nerve.

Receives rich blood supply from inferior thyroid artery and from anastomosis between superior and inferior thyroid arteries.

Veins and lymphatic’s of the gland are associated with those of thyroid gland, i.e., inferior thyroid and middle thyroid vein.

Pre and paratracheal group of lymph nodes.

Vasomotor nerves are derived from middle and superior cervical sympathetic ganglion.

It is controlled by blood calcium levels – low levels – stimulate and high level inhibits the activity of the gland.

Applied Anatomy:

1. Tumours of para thyroid glands – lead to excessive secretion of parathormone – cause increased removal of calcium from bones making them weak and liable to fracture.

Hypercalcaemia and increased urinary excretion of calcium salts lead to formation of stones in urinary tract.

2. Hypoparathyroidism – may occur spontaneously or from accidental removal of glands during – thyroidectomy – results in hypocalcaemia leading to increased neuromuscular irritability causing muscular spasm and convulsions (Tetany).

Term Paper # 7. Thymus Gland:

It is an important lymphatic structure and essential for immunity of the body.

This is well developed at birth, continues to grow upto puberty, thereafter atrophy and replaced by fat.

It develops from the endoderm of the third pharyngeal pouch.

Superior mediastinum and upper part of anterior mediastinum, extending above into the lower part of the neck.

At the time of birth – 10 to 13 gms.

Greatest development during first two years of life.

Adult – It regresses after puberty and gets atrophied in the adult. It weighs only less than 10 gms.

A pair of lobes united by connective tissue.

Cervical part of the thymus is usually rudi­mentary.

Upper end of each lobe extends upto the thyroid gland and lower end extends upto the 4th costal cartilage. It lies anterior to great vessels, peri­cardium and trachea.

Anterial Supply:

1. Internal thoracic artery.

2. Inferior thyroid artery.

Venous Drainage:

1. Left brachiocephalic vein.

Lymphatic Drainage of Thymus:

1. Brachiocephalic lymph nodes.

2. Tracheo bronchial lymph nodes.

3. Parasternal group of lymph nodes.

Sympathetic- Cervicothoracic ganglion.

Phrenic nerve- Supplies the capsule of the gland.

1. Lymphoid organ – controls lymphopoiesis and maintains an effective part of circulating lymphocytes.

2. Organ of immunogenesis – immunologically competent lymphocytes is produced.

3. Abnormality of thymic functions may pro­duce – autoimmune disorders like – myasthenia gravis, haemolytic anaemia, hashimotos thyroiditis.

4. Thymosin is a hormone secreted by gland – acts on lymph nodes to produce lymphocytes.

5. It produces T-lymphocytes.

6. It controls development of peripheral lymphoid tissues of the body during neonatal period. By puberty – main lymphoid tissues are fully developed.