The functional unit of the thyroid gland is the follicle ( Fig. 7.2 ).

Thyroid follicles are composed of a single layer of epithelial cells (thyroid follicular cells) surrounding a central space filled with colloid.

The follicular cells synthesize Tg, a large tyrosine-rich glycoprotein, and secrete it into the lumen of the follicle; colloid is essentially a pool of Tg.

Key thyroid hormones include thyrotropin-releasing hormone (TRH), thyroid-stimulating hormone (TSH), T₄, and T₃.

Only 1% of total thyroid hormone is in the unbound or free state and available for metabolic purposes. The rest is bound to globulin, prealbumin, and albumin.

Calcitonin is a peptide produced by the parafollicular cells of the thyroid gland. It reduces resorption of calcium in the bone and lowers the serum calcium.