Thyroid Hormone Physiology and Regulation

The thyroid produces thyroxine (T4) and triiodothyronine (T3), hormones that set the metabolic pace of nearly every organ system. They influence heat production, heart rate, bowel activity, neuromuscular function, growth, and energy use. Too little hormone slows these systems; too much pushes them into overdrive. Regulation follows a three-level negative-feedback loop: 1. The hypothalamus releases thyrotropin-releasing hormone (TRH). 2. TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH). 3. TSH stimulates the thyroid gland to produce and release T4 and T3. As circulating thyroid hormone rises, the pituitary reduces TSH secretion. When thyroid hormone falls, TSH rises to stimulate more production. TSH is therefore a useful signal of what the pituitary senses. In primary thyroid disease, the problem is in the thyroid gland itself. A failing thyroid produces less hormone, so TSH rises. An overactive thyroid produces excess hormone, so TSH becomes suppressed. The feedback loop explains a common laboratory pattern: - Primary hypothyroidism: high TSH with low free T4. - Primary hyperthyroidism: low TSH with high free T4 and/or T3. Pituitary or hypothalamic disease can disrupt this pattern....