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Hypothyroidism and thyroiditis: Clinical
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TSH then binds to TSH receptors, which makes the thyroid secrete thyroxine, or T4 and triiodothyronine, or T3, in the blood. But this is not a one-way street - there’s also negative feedback happening, meaning when thyroid hormone levels rise, that inhibits the production of TSH and TRH, halting further production of T3 and T4 - to keep everything in balance.
Normally, some thyroid hormones travel through the bloodstream bound to thyroid binding globulin, and some are in their free form. And the thyroid actually makes more T4 than T3, and that T4 is converted to the more potent T3 in the periphery.
Thyroid hormones then increase the rate of metabolism in all cells, so they make us think, move, and talk faster, and they also increase heat generation. They also activate the sympathetic nervous system, the part of the nervous system responsible for our ‘fight-or-flight’ response, increasing cardiac output.
So with hypothyroidism, it’s like the entire body is functioning in slow motion - but this happens gradually, so it can take years before symptoms are even recognized. These include weight gain despite a loss in appetite because of the lower basal metabolic rate; cold sensitivity because the body is producing less heat; and slower heart rate, mental slowness, lethargy, and constipation because of the decreased effect of thyroid hormones on the sympathetic nervous system.
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