There are three main Thyroid hormones that exert biological activity; T3 (3',5,3-L-triiodothyronine), T4 or Thyroxine (3',5',3,5-L-tetraiodothyronine) and T2 (3,5',-L-diiothyronine). The thyronine molecules known as reverse T3 (rT3, or 3',5',3-L-triiodothyronine) and 3,5,-diiodothyronine also exert some thyroid-like effects.
Synthesis of thyroid hormones is the only known bodily function of dietary iodine.
Thyroid hormones are synthesized in the thyroid gland of mammals. The cells of the thyroid gland take up the essential mineral iodine and form a protein known as thyroglobulin with the amino acid L-Tyrosine.
Thyroid hormones are typically regulated via free thyroid hormones and protein bound thyroid hormones, with the former regulated in a picomolar concentration and the latter in a greater (nanomolar) concentration.
The circulating levels of free thyroid hormones are generally seen as causative of the effects of thyroid hormone. The average level of overall (protein bound + free) thyroid hormones in adult vertabraes is about 91.6-115 nanomoles for T4 and 1.9-2.2 nanomoles for T3 while the average levels of free thyroid hormones is 40-50 picomoles (with ranges of 14.3-16.4pM and 4.1-6.4pM for T4 and T3 respectively). This gives a free thyroid hormone content of 0.01-0.1% on average.
Somebody classified as euthyroidic would have a T4 range of 60-140nM, a T3 range of 1-3nM, free T4 of 10-25pM and free T3 of 3-8pM.