Thyroid gland

The thyroid gland lies in the front of the neck just below the larynx, with a left and a right lobe on either side of the trachea joined by a thin bridge of tissue, the isthmus. It has a very rich blood supply. The small parathyroid glands, which control blood calcium, are usually embedded in the back of its lobes. Inside, the thyroid is made of many hollow spheres called follicles, each lined by follicular cells and filled with a protein-rich gel, the colloid.

Follicular cells take up iodide from the blood and attach iodine to a protein called thyroglobulin, building the hormones thyroxine (T4, with four iodine atoms) and triiodothyronine (T3, with three). Most of the output is T4, which tissues convert to the more potent T3. These hormones raise the basal metabolic rate and heat production and are essential for normal growth and brain development. Their release is driven by thyroid-stimulating hormone from the pituitary, itself controlled by the hypothalamus, in a negative feedback loop. Scattered parafollicular (C) cells secrete calcitonin, which can lower blood calcium.

A lack of dietary iodine leads to low hormone levels and an enlarged gland, a goiter. Where iodine is adequate, hypothyroidism is most often due to autoimmune Hashimoto thyroiditis, and hyperthyroidism to Graves disease, in which antibodies overstimulate the gland.