Thyroid Gland Anatomy and Physiology

The thyroid gland is situated just below the larynx, with its right and left lateral lobes lying on either side of the trachea, connected together by the isthmus. It plays a major role in the metabolism, growth and development of the human body, regulating body functions by constantly releasing a steady amount of thyroid hormones into the bloodstream. It receives a rich blood supply of about 80 to 120ml per minute.

Thyroid Gland Histology

The thyroid gland is composed of spherical sacs called thyroid follicles which are covered by a wall made up of Follicular Cells and Parafollicular Cells a.k.a. C Cells.

• Follicular Cells produce thyroxine (T₄) and triiodothyronine (T₃). Whilst T₄ is usually produced in greater quantities than T₃, T₃ is up to 4 times more potent than T₄. Additionally, about a third of T₃ is converted to T₄ within peripheral tissues, especially within the lungs and the liver.
• Parafollicular Cells produce calcitonin (CT).

Thyroid Hormone Formation, Storage & Release

The thyroid gland is the only endocrine gland that stores its hormonal products in large quantities, eventually releasing them steadily over time. Thyroid hormone formation, storage and release occurs through the following process:

1. Iodide Trapping – iodide ions are actively transported from the blood to the follicular cells
2. Thyroglobulin Synthesis – during iodide ion trapping, follicular cells synthesise the glycoprotein Thyroglobulin (TGB) which is released into the thyroid follicle lumen by exocytosis, resulting in colloid accumulation within the lumen and Tyrosine (amino acids) iodination in TGB
3. Iodide Oxidation – iodide ions bind to TGB following oxidation; simultaneously, iodine is formed by the action of peroxidase
4. Tyrosine Iodination – formed iodine reacts with tyrosine in the colloid; one iodine atom binding forms monoiodotyrosine (T₁); a second iodine atom binding produces diiodotyrosine (T₂)
5. T₁ and T₂ Coupling – T₁ and T₂ join and form thyroid hormones
6. Colloid Pinocytosis & Digestion – colloid droplets re-enter the follicular cells though pinocytosis, and then merge with lysosomes in the follicular cells; lysozyme breaks down TGB, and then produce T₃ and T₄ molecules
7. Thyroid Hormone Secretion – lipid-soluble T₃ and T₄ diffuse through the plasma membrane into the interstitial fluid, and then into the blood; T₄ is secreted in larger quantities than T₃, yet T₃ is much more potent than T₄
8. Transport into Blood – Thyroxine-Binding Globulin (TBG) which is a transport protein found within the blood plasma combine with both T₃ and T₄ and are carried around in the body within the blood; when T₄ enters a cell, most of it is converted to T₃ following removal of one iodine

NOTE: Iodine supplements may be given to pregnant women and for compensation of hypothyroidism.

Thyroid Hormone Regulation

Thyroid hormone secretion is stimulated by various factors…

Calcitonin (CT)

Calcitonin, which is produced by the parafollicular cells of the thyroid gland, is involved in the homeostasis of blood calcium level:

• Calcitonin inhibits bone breakdown and promotes bone calcium absorption
• Calcitonin is used in the treatment of post-menopausal osteoporosis along with calcium and vitamin dietary intake
• Calcitonin secretion is controlled via a negative feedback system

NOTE: Diarrhoea is a possible sign of increased thyroid hormone. Similarly, constipation is a possible sign of underactive thyroid.

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Claire

Claire Galea is a mum of three currently in her final year following a Degree in Nursing at the Faculty of Health Sciences, University of Malta, as a mature student.

Claire is keen about public education on health-related subjects as well as holistic patient-centered care. She is also passionate about spreading awareness on the negative effects that domestic abuse leaves on its victims’ mental, emotional, social and physical wellbeing.

Claire aspires to continue studying following completion of her Nursing Degree, because she truly believes in lifelong education.

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Author: Claire

Claire Galea is a mum of three currently in her final year following a Degree in Nursing at the Faculty of Health Sciences, University of Malta, as a mature student. Claire is keen about public education on health-related subjects as well as holistic patient-centered care. She is also passionate about spreading awareness on the negative effects that domestic abuse leaves on its victims’ mental, emotional, social and physical wellbeing. Claire aspires to continue studying following completion of her Nursing Degree, because she truly believes in lifelong education. View all posts by Claire