Thyroxine (T4) is one of two major hormones produced by the thyroid gland, a small butterfly-shaped organ that lies flat across the windpipe at the base of the throat. The other major thyroid hormone is called triiodothyronine (T3) and together they help control the rate at which the body uses energy. Almost all of the T4 (and T3) found in the blood is bound to protein.
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