Physiology, Thyroid

Excerpt

The thyroid gland is composed of thyroid follicles that synthesize and store thyroid hormone. The epithelial cells referred to as follicular cells or thyrocytes surround the colloid in the lumen. The ultimo-branchial cells or neural cells accompanying them are the origins of the C-cells in the thyroid gland, which secrete the hormone calcitonin.

The hypothalamus releases thyroid-releasing hormone (TRH), which stimulates thyrotrophs of the anterior pituitary to secrete thyroid-stimulating hormone (TSH). The anterior pituitary releases TSH and stimulates the thyroid follicular cells to release thyroxine, T4 (80%), and triiodothyronine or T3 (20%). The synthesis of thyroid hormones is dependent on the availability of iodide, TSH stimulation, and tyrosine residues on thyroglobulin (TG). When T4 is released into circulation, it can convert to T3 through the process of deiodination. T4 and T3 can then exert negative feedback on TSH levels with high levels of T3/T4, decreasing TSH, and low levels of T3/T4 increasing TSH levels from the anterior pituitary. This article reviews the physiology, biochemistry, and clinical relevance of thyroid hormones.