Thyroid hormone

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). An important component in the synthesis is iodine.

Related Topics:
Tyrosine - Hormone - Thyroid gland - Basal metabolic rate - Protein synthesis - Catecholamine - Adrenaline - Iodine

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The major form of thyroid hormone in the blood is thyroxine (T4). This is converted to the active T3 within cells by deiodinases.

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Most of the thyroid hormone circulating in the blood is bound to transport proteins :

Related Topics:
Blood - Protein

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• Thyroxine-binding globulin (TBG)
• Thyroid binding prealbumin (TBPA) - this protein is also responsible for the transport of retinol, and so now has the preferred name of transthyretin (TTR)
• albumin

Only a very small fraction of the circulating hormone is free (unbound) - T4

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0.03% and T3 0.3%. This free fraction is

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biologically active, hence measuring concentrations of free thyroid hormones is

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of great diagnostic value. These values are referred to as fT4 and fT3. Another critical diagnostic tool is the amount of thyroid-stimulating hormone that is present.

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When thyroid hormone is bound, it is not active, so the amount of free T3/T4 is what is important. For this reason, measuring total thyroxine in the blood can be misleading.

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The thyroid hormones are essential to proper development and differentiation of all cells of the human body. To various extents they regulate protein, fat and

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carbohydrate metabolism. But they have their most pronounced

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effects on how human cells use energetic compounds.

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Numerous physiological and pathological stimuli influence thyroid hormone synthesis.

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Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.

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Thyroid hormone: Structure and production of the thyroid hormones ►