Bianco – Using Serum TSH as Biological Index of Thyroid Function

The negative feedback loop between circulating thyroid hormone and the neuroendocrine system (hypothalamus and pituitary gland) is the fundamental mechanism preserving thyroid hormone homeostasis. During iodine deficiency, there is decreased synthesis of thyroid hormone that leads to a reduction in plasma thyroxine (T4), rapidly activating secretion of hypothalamic thyrotropin-releasing hormone (TRH) and pituitary thyroid-stimulating hormone (TSH), which stimulates the thyroid to grow and normalize thyroid hormone synthesis and secretion. As with T4, a reduction in plasma 3,5,3′-triiodothyronine (T3), the biologically active thyroid hormone, also triggers the feedback mechanism and activates TRH and TSH secretion. A focus will be placed on understanding the appropriate tests required to diagnose hypothyroidism and how to restore hormonal balance through evidence based therapy with combination therapy including levothyroxine and liothyronine.