provide reviews of the thyroid axis and its disorders. The thyroid gland in the human consists of two lateral lobes (connected by an isthmus) that lie over the trachea below the cricoid cartilage and is one of the largest endocrine glands, weighing about 20-30 g in an adult. Arterial blood is supplied by the superior thyroid artery,' arising from the external carotid arteries, and the inferior, arising from the subclavian arteries. Autonomic innervation is provided by both adrenergic and cholinergic neurons arising from the cervical ganglia and the vagus nerve , respectively. Neurogenic-adrenergic innervation regulates blood flow through the gland in addition to acting directly at the level of the follicle cell .
Synthesis and Metabolism of Thyroid HormonesSynthesis of thyroid hormones is dependent on the availability of adequate quantities of exogenous iodine. Iodine is provided by inorganic iodide from the diet, iodide leak (passive loss of iodide from the thyroid into the blood), and peripheral deiodination of thyroid hormones. Iodide is concentrated in extracellular fluid , from which it is drawn during the synthesis of thyroid hormones. Iodide is stored in red blood cells and intraluminal fluids such as saliva and gastric juice and is cleared predominately through the thyroid and kidney, which reabsorbs iodide passively and is not considered as sharing in thyroid homeostasis. The major channel for removal of iodide from extracellular fluid is the thyroid gland. The thyroid gland is unique among endocrine glands in that it has a large store of hormone and a slow rate of turnover. This large reservoir (about 100 days of the average output of the thyroid gland) provides extended protection against depletion of circulating thyroid hormone in the event of termination of synthesis of th yroid hormones. BILL E. BECKWITH • She ll Point Retir ement Comm unity. Fort Myers, Florida 33908.