Light micrograph (LM) of Thyroxine crystals. The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland. They act on the body to increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline). The major form of thyroid hormone in the blood is thyroxine. This is converted to the active T3 within cells by deiodinases. Most of the thyroid hormone circulating in the blood is bound to transport proteins: such as thyroid binding globulin (TBG). Only a very small fraction of the circulating hormone is free (unbound). This free fraction is biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value. Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine and free triiodothyronine, or both. Magnification: x80 when shortest axis.
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