How is thyroid hormone production controlled?
In the follicular cells of the thyroid, a substance called thyroid peroxidase reacts with iodine to form T4 (Thyoxine) and T3(triiodothyronine). Iodine is found in the soil, seawater and some foods, but may not be adequate in some places. In some countries, iodine needs to be added to foods such as salt. T4 & T3 are stored in the thyroid as a molecule called thyroglobulin until needed. T4 and T3 work together to control the metabolic rate of the body or generate energy, heat for muscle and brain function, regulate heart rate, and maintain bones. However, T3 is the most active thyroid hormone in body cells.
When body tissue levels are low, thyroglobulin is broken down into T4 (and some T3) and secreted from the follicular cells into the blood. However, T4 needs to be converted to T3 in peripheral tissues such as in the liver and kidneys and brain and some fat tissue. T3 then circulates in the blood, attaches to receptors in a lock and key fashion and is transported into the target cell nucleus where it performs its function.
After available T4 and T3 have been used, low levels of both signal the cycle to restart, the hypothalamus and the pituitary to release more TRH and TSH and in turn, the follicular cells of the thyroid to make more T4 & T3. This completes a feedback loop.
When a tumor damages the hypothalamus and/or the pituitary, the production of TRH or TSH may be disrupted, causing low thyroid levels. This is known as central or secondary hypothyroidism.
Image: Feedback mechanism in the control of thyroid hormone production.