The thyroid is an organ in the neck overlying the trachea. It is responsible for releasing thyroid hormones T3 and T4, which play a major role in metabolism. Too much tends to speed up the metabolism and too little tends to slow the metabolism down. A number of mechanisms exist to control the amount of T3 and T4 reaching the cells of the body. TSH is the signaling protein released by the pituitary gland that affects iodide metabolism, thyroid hormone synthesis and secretion.
Increased Iodine Metabolism
Iodide is a crucial component of the thyroid hormones. According to "Williams Textbook of Endocrinology," TSH affects iodide metabolism in four different ways. It increases iodide in the cavities and in the cells of the thyroid, increases the number of transporters in the thyroid that take in sodium and iodide, and increases blood flow to the thyroid.
Augmented Thyroid Hormone Synthesis
TSH also increases thyroid hormone synthesis by affecting many of the steps of the synthetic pathway. For example, it increases hydrogen peroxide, synthesis of thyroglobulin and thyroid peroxidase, and NADPH, a chemical used to build molecules such as thyroid hormone.
Increased Thyroid Hormone Secretion
TSH increases the release of thyroid hormone in at least three ways. It increases the uptake of stored thyroid hormone and its release into the general circulation, and causes cellular division in the thyroid, causing it to grow larger.
Effects on Hormone Balance
As the crucial hormone that signals the thyroid to release more thyroid hormone, TSH plays an important role in the overall balance of thyroid hormone in the body. If the level of thyroid hormones are too high, less TSH will be released by the pituitary, and the result is less synthesis of thyroid hormones. Alternatively, if the level of thyroid hormone is too low, more TSH is released by the pituitary and more thyroid hormone will be synthesized. Imbalances in levels of TSH can lead to hypothyroid or hyperthyroid states, but this is not the only mechanism.
References
- "Williams Textbook of Endocrinology"; Henry M. Kronenberg; 11th Ed 2008
- Anaesthetist.com: Thyroid Physiology and Tests of Function
