The hormones of thyroid glands are there in number:
*Thyroxine (tetraiodothyronin or T4)
*Triiodothyronine (T3)
*Calcitonin
Iodine joins with the amino acid thyronine and diiodithyronine, and is part of triiodothyronine (T3), thyroxine (T4), and thyroglobulin (the storage form of thyroid hormones). These hormones (T3 and T4) are under control of thyroid, pituitary, brain, and feedback messages from peripheral tissues.
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| Triiodothyronine (T3) |
In the peripheral circulation, thyroxine will lose one of its iodine molecules and become triiodothyronine, the more potent of the thyroid hormones.
In triiodothyronine, iodine atoms are attached to 3 and 5 positions of the inner benzene ring and only 3’ position of the outer benzene ring.
Low T3 levels are common in severe illness, especially in the elderly. This is called euthyroid syndrome or non thyroidal illness syndrome.
Triiodothyronine hormone
