Regulation of Thyroid Hormones
The thyroid gland concentrates iodine that the body absorbs from various foods to make thyroid hormones. The thyroid gland makes mostly T4. Very little T3 is made by the thyroid gland. The liver and other organs convert T4 into T3. Certain diseases and drugs can affect the conversion of T4 into T3.
Both T4 and T3 circulate in the blood, mostly bound to certain proteins. Thyroid hormones become active in their role of controlling the speed of vital body functions when they are freed from protein binding. The body regulates a balance between bound thyroid hormones and free thyroid hormones.
Through a complex interaction between the hypothalamus (a part of the brain) and the pituitary gland (located inside the skull, just underneath the brain), the thyroid gland knows how much of its hormones to produce. The hypothalamus makes thyrotropin-releasing hormone, which then stimulates the pituitary gland. The pituitary gland responds by making another hormone called thyroid-stimulating hormone, or TSH. As the name suggests, thyroid-stimulating hormone stimulates the thyroid gland to make hormones. If the level of thyroid hormones in the blood climbs above what is needed, the pituitary gland makes less thyroid-stimulating hormone. The thyroid gland then cuts back on making thyroid hormones. If the level of thyroid hormones in the blood falls too low, the pituitary gland makes more thyroid-stimulating hormone. In response, the thyroid gland makes and secretes more thyroid hormones. The body adjusts the amount of thyroid hormones to suit its needs in the same way a thermostat adjusts the temperature in a house.
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