Thyroid Hormone Secretion

Over 90% of the active hormone secreted by the thyroid gland is thyroxine (T4) and the rest is triiodothyronine (T3).

The functions of these two hormones are essentially the same however there is a difference in their intensity of action (T3 is approximately four times more active).

Most of the T4 is converted into T3 within the tissues although both are functional.