The Thyroid Gland

The Thyroid Gland

The thyroid gland is located at the front of the neck just below the Adam’s apple. It has two lobes and the gland is butterfly shaped. The thyroid is one of the largest endocrine glands, weighing up to 60 grams in adult humans.

Thyroid Hormone Synthesis and Secretion

The thyroid is largely composed of a collection of spherical structures known as follicles. Specialised follicular cells called epithelial cells surround each follicle. Within each follicle is a colloid (a semi-liquid gelatinous substance). The follicular space and the colloid that it contains serve as a reservoir for all the materials required for producing thyroid hormones. It is within each of these thyroid follicles that much of the activity leading to the production of thyroid hormones occurs.


Amongst the thyroid epithelial cells are other specialised cells that produce calcitonin. Calcitonin is a hormone produced by the parafollicular cells.

Calcitonin acts to reduce blood calcium levels and works opposite to parathyroid hormone (PTH) from the parathyroid glands.

Calcitonin reduces calcium absorption by the gut, inhibits the break down of calcium within the bones, increases urinary excretion of calcium and regulates vitamin D.

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