Your thyroid gland is responsible for regulating a number of bodily functions, including your metabolism (energy production), body temperature, growth and repair, muscle contraction and digestion processes. The thyroid cells are the only cells in the body which can absorb iodine. They combine iodine with the amino acid tyrosine to produce two main thyroid hormones: thyroxine (T4) and its active counterpart, triiodothyronine (T3), which are then released into the bloodstream.
T3 is critical to making every system in your body work at the right speed. Although T3 is the more active thyroid hormone, the thyroid gland produces much more T4 than T3. T4 is converted to T3 when needed. Too much T3 will cause enzymes to convert it into reverse T3 (rT3), an inactive form. If you produce too much or too little thyroid hormone, your whole body will be affected. Balance is key.
Thyroid hormone balance is maintained via negative feedback. When thyroid hormone levels fall too low, (1) the hypothalamus (in the brain) produces thyrotropin-releasing hormone (TRH) which stimulates the pituitary gland (also in the brain) to (2) release thyroid stimulating hormone (TSH). TSH stimulates the thyroid too (3) produce more thyroid hormones (T4 and T3). As soon as levels increase again, the pituitary responds by (4) decreasing the production of TSH.
Genes play a vital role in regulating this mechanism. Studies into genetic inheritance suggest that up to 67% of circulating TSH and thyroid hormone levels are genetically determined.
