The histology of thyroid gland is a description of the essential tissue that constitutes this endocrine organ. The thyroid is composed of a collection of millions of sac-like cavities of various sizes; these are called follicles. The inner walls of these follicles consists in turn of a single layer of glandular epithelial cells, which are normally cuboidal and columnar when they are very active, and flat when inactive. The structure of the gland also includes connective tissue stroma, which is made up of collagenous and elastic fibers enmeshed in a network of tiny blood vessels and nerves.
The epithelial cells lining each follicle produce thyroglobulin, which is released into the lumen of the follicle. The biosynthesis of thyroid hormones takes place at the cell colloid interface; this is where thyroglobulin is hydrolized to release thyroid hormones: thyroxine and triiodothyronine. Aside from the follicular cells, there are light appearing cells, which are often found in clusters between the follicles; these are called C-cells, which derive from the neural crest and secrete calcitonin. In adults, these C-cells represent only 1% of all the thyroid cells.
The normal growth and development of the body and tissue depend on the normal functioning of the thyroid gland, which secretes two types of hormones: thyroxine (T4) and triiodothyronine (T3). Thyroxine is produced by iodination (addition of iodine atoms in organic compounds) of the amino acid tyrosine and the oxidative condensation of two diiodotyrosine molecules, with the splitting off of alanine. Triiodothyronine, on the other hand, is synthesized by the oxidative condensation of monoiodotyrosine and diiodotyrosine.
