6.2.2 Thyroid Gland

The thyroid gland is a butterfly-shaped endocrine gland located in the neck. It produces three active hormones: triiodothyronine (T₃), tetraiodothyronine (T₄) or thyroxine, and calcitonin. These hormones regulate metabolism, growth, and calcium balance.

Definition

Thyroid Gland: Thyroid gland is composed of two lobes which are located on either side of the trachea inferior to the larynx. Thyroid gland produces three active hormones, triiodothyronine (T₃), tetraiodothyronine (T₄) or thyroxin, and calcitonin.

Main Idea

• Thyroid gland is composed of two lobes.
• Located on either side of the trachea inferior to the larynx.
• Produces three active hormones:
• T₃ and T₄ are iodine-containing hormones.
• TSH from anterior pituitary stimulates production and release of T₃ and T₄.
• Calcitonin is produced by C-cells.
• Calcitonin regulates calcium balance.

Core Concept

• Thyroid gland = two lobes; on either side of trachea
• Hormones = T₃, T₄ (thyroxin), calcitonin
• T₃ = triiodothyronine; three iodine atoms
• T₄ = tetraiodothyronine or thyroxin; four iodine atoms
• **TSH** = stimulates T₃ and T₄ production
• Calcitonin = from C-cells; regulates calcium
• **Hyperthyroidism** = excess T₃ and T₄
• Hypothyroidism = deficiency of thyroxin
• Graves' disease = over secretion of T₃ and T₄
• Myxedema = adult hypothyroidism
• Cretinism = congenital hypothyroidism

Detailed Explanation

Thyroid Gland

• Thyroid gland is composed of two lobes which are located on either side of the trachea inferior to the larynx.
• Thyroid gland produces three active hormones, triiodothyronine (T₃), tetraiodothyronine (T₄) or thyroxin, and calcitonin.

T₃ and T₄

• These are iodine containing hormones.
• Triiodothyronine contains three iodine atoms in structure and thyroxin contains four, hence the names T₃ and T₄.
• TSH from anterior pituitary stimulates production and release of these hormones.

• They promote basal metabolic rate of the body.
• Enhance glucose catabolism and synthesis of cholesterol in the liver.
• Promote development of nervous system in foetus and infants.
• They act on muscles for their development and functioning.
• Promote growth and maturation of skeleton.
• These hormones also promote normal motility of the gastrointestinal tract.

• Term applied to excess of these hormones.
• Over secretion of T₃ and T₄ causes Graves' disease.

• T₄ (also known as thyroxin) is the major hormone, about 90%, secreted by the thyroid; T₃ is only 10%.
• T₃ is four times more potent than T₄, however, action duration of T₄ is four times more than T₃.

• Graves' disease is believed to be an autoimmune disease.
• The serum of patients contains abnormal antibodies that mimic TSH and continuously stimulate thyroxin release.
• The symptoms include:
• High metabolic rate
• Rapid and irregular heartbeat
• Increased breathing rate
• Increased body temperature
• Sweating
• Weight loss despite adequate food intake
• Mostly exophthalmia (protrusion of the eyeballs) results from Graves's disease and is a classic symptom of hyperthyroidism.

• Is the under secretion of thyroxin.
• In adults, the full-blown hypothyroid syndrome is called myxedema.
• Myxedema is characterized by:
• Low metabolic rate
• Feeling chilled
• Puffy eyes
• Thick and dry skin with hair lost from the scalp and eyebrows
• Oedema
• Tongue swelling
• Constipation
• Enlarged thyroid gland i.e., goiter
• Myxedema may result due to deficiency of iodine in diet.
• Congenital under secretion results in a severe hypothyroidism in infants called cretinism.
• Cretinism is characterized by:
• Mental retardation with poor physical growth
• Disproportionate body size
• Bone maturation and puberty are severely delayed
• Infertility is common

Calcitonin

• The C-cells in thyroid gland produce calcitonin.
• Excessive calcium level of blood stimulate release of calcitonin whereas declining blood calcium levels inhibit its secretion.
• Calcitonin increases the deposition of calcium in bone matrix.
• Calcitonin inhibits calcium absorption by the intestine and decreases its reabsorption by the kidney tubules allowing its excretion in urine.

Science Titbits

• Renin monitors blood pressure and takes corrective action if it drops.
• Erythropoietin acts on the bone marrow to increase the production of red blood cells. Stimuli such as bleeding or moving to high altitudes (where oxygen is scarcer) trigger the release of EPO.
• Calcitriol acts on the cells of the intestine to promote the absorption of calcium from the diet.

Summary Table of Thyroid Hormones

Disorders of Thyroid Gland

Intermediate Level

Thyroid Hormones

Physiological Effects of T₃ and T₄

Disorders of Thyroid Gland

Calcitonin

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Thyroid gland is composed of two lobes.
• 2. Located on either side of the trachea inferior to the larynx.
• 3. Produces T₃, T₄, and calcitonin.
• 4. T₃ has three iodine atoms.
• 5. T₄ has four iodine atoms.
• 6. TSH stimulates T₃ and T₄ production.
• 7. T₃ and T₄ promote basal metabolic rate.
• 8. T₃ and T₄ enhance glucose catabolism.
• 9. T₃ and T₄ promote nervous system development.
• 10. T₃ and T₄ promote muscle development.
• 11. T₃ and T₄ promote skeleton growth.
• 12. T₃ and T₄ promote GI motility.
• 13. Over secretion of T₃ and T₄ causes Graves' disease.
• 14. Graves' disease is an autoimmune disease.
• 15. Symptoms of Graves' disease include exophthalmia.
• 16. Under secretion of thyroxin causes myxedema in adults.
• 17. Congenital under secretion causes cretinism.
• 18. Calcitonin is produced by C-cells.
• 19. Calcitonin increases calcium deposition in bone.
• 20. Calcitonin inhibits calcium absorption and reabsorption.

Remember

• - Thyroid gland = two lobes; on either side of trachea
• - Hormones = T₃, T₄ (thyroxin), calcitonin
• - T₃ = triiodothyronine; three iodine atoms
• - T₄ = tetraiodothyronine or thyroxin; four iodine atoms
• - **TSH** = stimulates T₃ and T₄ production
• - Calcitonin = from C-cells; regulates calcium
• - **Hyperthyroidism** = excess T₃ and T₄
• - Hypothyroidism = deficiency of thyroxin
• - Graves' disease = over secretion of T₃ and T₄
• - Myxedema = adult hypothyroidism
• - Cretinism = congenital hypothyroidism

Common Mistakes

• 1. Confusing T₃ and T₄ — T₃ has three iodine atoms; T₄ has four.
• 2. Mixing up the functions — T₃ and T₄: metabolism; calcitonin: calcium.
• 3. Forgetting the role of TSH — Stimulates T₃ and T₄ production.
• 4. Confusing hyperthyroidism and hypothyroidism — Hyper = excess; hypo = deficiency.
• 5. Mixing up Graves' disease and myxedema — Graves' = hyperthyroidism; myxedema = hypothyroidism.
• 6. Forgetting the role of calcitonin — Increases calcium deposition in bone.
• 7. Mixing up the sources — T₃ and T₄ from thyroid; calcitonin from C-cells.
• 8. Thinking goiter is only caused by iodine deficiency — It can also be caused by other factors.
• 9. Believing exophthalmia is a symptom of hypothyroidism — Exophthalmia is a symptom of hyperthyroidism (Graves' disease).
• 10. Mixing up cretinism and myxedema — Cretinism = congenital; myxedema = adult.

Quick Revision

Thyroid gland is composed of two lobes which are located on either side of the trachea inferior to the larynx. Thyroid gland produces three active hormones, triiodothyronine (T₃), tetraiodothyronine (T₄) or thyroxin, and calcitonin. T₃ and T₄ are iodine containing hormones. Triiodothyronine contains three iodine atoms in structure and thyroxin contains four, hence the names T₃ and T₄. TSH from anterior pituitary stimulates production and release of these hormones. These hormones show a variety of physiological effects: (1) They promote basal metabolic rate of the body. (2) Enhance glucose catabolism and synthesis of cholesterol in the liver. (3) Promote development of nervous system in foetus and infants. (4) They act on muscles for their development and functioning. (5) Promote growth and maturation of skeleton. (6) These hormones also promote normal motility of the gastrointestinal tract. Hyperthyroidism term is applied to excess of these hormones. Over secretion of T₃ and T₄ causes Graves' disease. Hypothyroidism is the under secretion of thyroxin. In adults, the full-blown hypothyroid syndrome is called myxedema. Congenital under secretion results in a severe hypothyroidism in infants called cretinism. The C-cells in thyroid gland produce calcitonin. Excessive calcium level of blood stimulate release of calcitonin whereas declining blood calcium levels inhibit its secretion. Calcitonin increases the deposition of calcium in bone matrix.

60-Second Revision

• Thyroid gland = two lobes; on either side of trachea
• Hormones = T₃, T₄ (thyroxin), calcitonin
• T₃ = triiodothyronine; three iodine atoms
• T₄ = tetraiodothyronine or thyroxin; four iodine atoms
• TSH = stimulates T₃ and T₄ production
• Calcitonin = from C-cells; regulates calcium
• Hyperthyroidism = excess T₃ and T₄
• Hypothyroidism = deficiency of thyroxin
• Graves' disease = over secretion of T₃ and T₄
• Myxedema = adult hypothyroidism
• Cretinism = congenital hypothyroidism
• Warning: T₃ has three iodine atoms; T₄ has four

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• Chapter:
• 6. Endocrine System of Man

• Topic:
• 6.2.3 PARATHYROID GLANDS

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