1. The endocrine system is a group of glands in the body that secrete chemical substances known as hormones directly into the circulatory system that transports them to the desired target organ. The thyroid gland is a major endocrine gland located in the lower part of the neck. The normal thyroid gland is a firm, smooth gland consisting of two lateral lobes and a connecting central isthmus. On a microscopic level, the thyroid gland entails numerous spherical hollow sacs known as thyroid follicles. These follicles are lined with a simple cuboidal epithelium composed of follicular cells that produce the principal thyroid hormone, thyroxin. The interior of the follicles contains colloid, a protein-rich fluid. The primary function of the thyroid is the synthesis and releases of the hormones triiodothyronine (T3), thyroxin (T4) and calcitonin which aid in the regulation of the body’s metabolic rate. Sufficient levels of thyroid hormone are necessary in infants for normal development of the CNS. In children for normal skeletal growth and maturation, and in adults for normal function of multiple organ systems. Most steps of thyroid hormone synthesis take place within the colloid. The thyroid captures iodide (I-) from the blood and transfers it into the colloid by an iodide pump. Once the iodide has entered the colloid, it is oxidized (reaction with Oxygen) and attached to a specific amino acid (tyrosine) within the polypeptide chain of a protein termed thyroglobulin. The structures of those tyrosines are heavily modified by enzymes in the colloid until thyroxin is eventually formed.
1. Hypothyroidism is a common endocrine