DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY DENNIS KUNKEL MICROSCOPY / SCIENCE PHOTO LIBRARY
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. The thyroid hormones are essential to proper development and differentiation of all cells of human body. To various extent they regulate protein, fat and carbohydrate metabolism. Magnification: x40 when shortest axis printed at 25 millimetres.
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