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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 thyroxine-binding globulin, and some are in their free form.
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.
Thyroid hormones are important - and the occasional increase is like getting a boost to fight off a zombie or to stay warm during a snowstorm!
But with hyperthyroidism, it’s like the entire body is buzzing at twice the normal rate. So individuals with hyperthyroidism tend to be hyperactive and talk really fast, and present with tremor in the extremities - as if they’ve had too much coffee. Also, making so much internal heat makes them sweat a lot and uncomfortable in warm temperatures.
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