Thyroid hormones
The thyroid hormones function via a well-studied set of nuclear receptors, termed the thyroid hormone receptors. These receptors, together with corepressor molecules, bind DNA regions called thyroid hormone response elements (TREs) near genes. This receptor-corepressor-DNA complex can block gene transcription. Triiodothyronine (T3), which is the active form of thyroxine(T4), goes on to bind to receptors. The deiodinase catalyzed reaction removes an iodine atom from the 5' position of the outer aromatic ring of Thyroxine's (T4) structure. When triiodothyronine (T3) binds a receptor, it induces a conformational change in the receptor, displacing the corepressor from the complex. This leads to recruitment of coactivator proteins and RNA polymerase, activating transcription of the gene.
Thyroid
An underactive thyroid gland results in hypothyroidism. Typical symptoms are abnormal weight gain, tiredness, constipation, heavy menstrual bleeding, hair loss, cold intolerance, and a slow heart rate. Iodine deficiency is the most common cause of hypothyroidism worldwide, and the autoimmune disease Hashimoto's thyroiditis is the most common cause in the developed world. Other causes include congenital abnormalities, diseases causing transient inflammation, surgical removal or radioablation of the thyroid, the drugs amiodarone and lithium, amyloidosis, and sarcoidosis. Some forms of hypothyroidism can result in myxedema and severe cases can result in myxedema coma.