The production and release of thyroid hormones, thyroxine and triiodothyronine, is controlled by a feedback loop system which involves the hypothalamus, the anterior pituitary gland and the thyroid.
The hypothalamus secretes a hormone, called thyrotropin-releasing hormone, which in turn, stimulates the pituitary gland to produce thyroid stimulating hormone. We will abbreviate these with T R H and T S H.
The T S H, the thyroid stimulating hormone, than stimulates the production of the thyroid hormones, thyroxine and triiodothyronine, by the thyroid gland.
T S H binds to the T S H receptor which is located on thyroid cells. The T S H receptor is a member of the G protein-coupled receptor superfamily and this basically means that it’s integrated in the cell membrane of the thyroid and stimulates intracellular activity. To be more specific, this process activates most functional aspects of the thyroid epithelial cell that we discussed in the previous video, including iodide pumping; thyroglobulin synthesis, iodination, endocytosis and proteolysis; thyroid peroxidase activity; and hormone release. If you have not seen the first video, click on this box to view it. Also a link to the video can be found in the description.
This hormone production system is regulated by a negative feedback loop so that when the levels of the thyroid hormones, T3 and T4, increase, they prevent the release of both thyrotropin-releasing hormone and thyroid stimulating hormone. This system allows the body to maintain a constant level of thyroid hormones in the body.
This is also the mechanism to prevent both hypo and hyperthyroidism, which means not enough or too much thyroid hormone production, respectively.
It should also be mentioned that cold temperatures also stimulate the release of thyrotropin releasing hormone by the hypothalamus, which means that the thyroid hormones will also be elevated to increase the basic metabolic rate. This causes the body to produce more heat to compensate the cold temperature.
Another factor that also affects this feedback loop is the amount of stress. During stress, more cortisol will be produced by the adrenal glands, which, on its turn, can decrease the amount of TSH released. So this decreases the amount of thyroid hormones in the end and signs of hypothyroidism may develop.
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