Chapter 33 – Thyroid Gland

The thyroid gland consists of two endocrine cell populations, the follicular cells and the C cells (parafollicular cells). The thyroid gland plays a crucial role in maintaining the optimal function of virtually all other organs. The follicular cells are responsible for the synthesis, storage, and secretion of thyroid hormones, which regulate the basal metabolic rate and the processes of growth and tissue differentiation. Calcitonin, produced by the C cells, helps regulate serum calcium and phosphate levels. Chemicals may affect the function of follicular cells by altering the pharmacokinetics, production, and utilization of thyroid hormones. Secretion of thyroid hormones is under feedback control by the pituitary. Prolonged stimulation of the follicular cells by a thyroid stimulating hormone (TSH) produced by the pituitary can lead to diffuse hyperplasia and neoplasia. The pituitary, which may be under prolonged stimulation to produce TSH, may also develop neoplasms. Similarly, biochemical, physiological, or structural changes to C cells may alter their function. Due to the central role of the thyroid gland in the overall health of an animal, it is commonly included as a protocol required tissue for morphological examination in most toxicity and carcinogenicity studies.