Thyroid Gland

The thyroid gland is the largest true endocrine gland in the human body. It secretes hormones that primarily regulate the metabolism of the body.

Origin of the Thyroid Gland

The thyroid gland originates from the endoderm of the cephalic portion of the alimentary canal of an embryo during the third or fourth week of development. It appears as a sac-like diverticulum on the ventral surface of the pharynx. After becoming a bilobed structure, it remains connected by the thyroglossal duct (not gland), which usually disappears.

Location of the Thyroid Gland

In humans, the thyroid gland is located anterior to the trachea, spanning from C5–T1 vertebral level, approximately, just below the cricoid cartilage (the hyaline cartilage encompassing the larynx).

Structure of the Thyroid Gland

The thyroid gland is a solid mass of bilobed structure that remains connected by a narrow isthmus. Each of the two oval lobes is situated on either side of the upper border of the trachea, with the isthmus on the ventral surface of the trachea. The pyramidal protrusion near the isthmus is believed to be the remnant of the atrophied thyroglossal stalk. The average weight of the human thyroid gland ranges from 15 to 20 grams.

Histology of the Thyroid Gland

The thyroid gland is made up of numerous spherical but irregular shaped thyroid follicles.
A single layer of cubical or columnar follicular cells lines each of the thyroid follicles, enclosing a colloidal lumen.
The space within the follicular wall and the extracellular space between the follicles are highly vascularised; connective tissues are also present.
In the same region, some specialised secretory cells, known as the parafollicular cells or C-cells, are also present, which contain a large number of mitochondria.
The thyroid gland is richly supplied with sympathetic postganglionic neurones that serve vasomotor functions.

Production of Hormones in the Thyroid Gland

Each thyroid follicle synthesises thyroglobulin and releases the protein into the colloidal matter via vesicular exocytosis.
In response to endocrine stimulation (TSH from the pituitary gland), follicular cells of a thyroid follicle engulf the colloid through phagocytosis.
Thyroglobulin is the substrate for tyrosine iodination, which is achieved by iodination and coupling of tyrosine residues within thyroglobulin to yield the thyroid hormones Thyroxine (T4) and Triiodothyronine (T3). The synthesis of the hormones (iodination + coupling in colloid) occurs in the lumen. Parafollicular cells secrete calcitonin (thyrocalcitonin).
The iodinated thyroid hormones are released from the follicular cells into the extracellular space, where they enter the capillaries and get taken up in the bloodstream.
Parafollicular cells secrete the peptide hormone Thyrocalcitonin (TCT).

Enlargement of the Thyroid Gland

Both hypothyroidism (reduced production of T4) and hyperthyroidism (elevated production of T4) can be associated with thyromegaly, in certain cases.

In hypothyroidism

Deficiency of dietary iodine leads to impaired synthesis of T4, which stimulates the pituitary to secrete more TSH (Thyroid Stimulating Hormone). Elevated TSH results in thyromegaly- a condition known as endemic goitre.

In hyperthyroidism

Presence of a tumour in the thyroid gland (high T4) or pituitary (high TSH) can result in the overgrowth of the thyroid gland, leading to thyrotoxicosis.

Functions of the Thyroid Gland

Metabolic Actions

T3 and T4 stimulate the oxidation of food in the cells, resulting in calorigenesis. During cold, the secretion of these hormones increases to induce calorigenesis and maintain thermoregulation. The hormones are also known as calorigenic hormones.
Thyroid hormones play an important role in carbohydrate metabolism- they elevate the blood sugar level through increased rate of glucose absorption, glycogenolysis, and gluconeogenesis.
At the optimum level, thyroid hormones help in protein synthesis, but hypersecretion of T3 and T4 causes protein breakdown.
The hormones activate the lipase enzyme and thus help in the catabolism of lipids.

Growth and Development

Thyroid hormones jointly act with pituitary hormone STH (Somatotrophic Hormone) to promote growth. STH directly stimulates the growth of a tissue, and T4 stimulates the differentiation of the tissue.
T4 and T3 play an important role in the development of organisms, examples including metamorphosis of amphibians, the moulting of reptiles, etc.

Action on Various Systems

T4 helps in the development of the nervous system; deficiency of the hormone results in intellectual disability.
Thyroid hormones increase the RBC production by stimulating erythropoietin release and enhancing metabolism, indirectly supporting hematopoiesis.
Hypothyroidism can cause infertility, delayed puberty; hyperthyroidism causes menstrual disturbances, so thyroid indirectly influences reproduction.
Thyroid hormones promote hunger, the movement of the intestinal tract, and the absorption of food.

Frequently Asked Questions (FAQs)

Q1. Which hormone does the thyroid gland produce apart from T3 and T4?

Apart from T3 (Triiodothyronine) and T4 (Thyroxine), the thyroid gland also secretes Calcitonin (Thyrocalcitonin) from parafollicular C-cells, which lowers blood calcium levels by inhibiting bone resorption.

Q2. What happens if the thyroid gland is underactive during childhood?

If the thyroid gland is underactive in childhood (congenital hypothyroidism), it leads to cretinism, a condition marked by stunted growth, delayed puberty, and severe intellectual disability.