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 hormones
Today most patients are treated with levothyroxine, or a similar synthetic thyroid hormone. Different polymorphs of the compound have different solubilities and potencies. Additionally, natural thyroid hormone supplements from the dried thyroids of animals are still available. Levothyroxine contains T4 only and is therefore largely ineffective for patients unable to convert T4 to T3. These patients may choose to take natural thyroid hormone, as it contains a mixture of T4 and T3, or alternatively supplement with a synthetic T3 treatment. In these cases, synthetic liothyronine is preferred due to the potential differences between the natural thyroid products. Some studies show that the mixed therapy is beneficial to all patients, but the addition of lyothyronine contains additional side effects and the medication should be evaluated on an individual basis. Some natural thyroid hormone brands are FDA approved, but some are not. Thyroid hormones are generally well tolerated. Thyroid hormones are usually not dangerous for pregnant women or nursing mothers, but should be given under a doctor's supervision. In fact, if a woman who is hypothyroid is left untreated, her baby is at a higher risk for birth defects. When pregnant, a woman with a low-functioning thyroid will also need to increase her dosage of thyroid hormone. One exception is that thyroid hormones may aggravate heart conditions, especially in older patients; therefore, doctors may start these patients on a lower dose and work up to a larger one to avoid risk of heart attack.