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The release of thyroid hormones into the blood is controlled by the hypothalamic-pituitary-thyroid (HPT) axis. Briefly:

  1. Cell bodies in the paraventricular nucleus of the hypothalamus detect low circulating levels of thyroid hormone and release TRH
  2. TRH acts on nearby anterior pituitary, releasing TSH
  3. TSH acts on the thyroid gland, causing the release of T3 and T4.
  4. An increase in the concentration of T3/T4 suppresses the release of both TRH and TSH.

My question is: what is the functional benefit of having both the hypothalamus and pituitary involved before the thyroid? Why not just have one or the other? It seems like a lot of energy is wasted in synthesising and releasing a second type of hormone that carries the same information as the first. Surely it would be better to just release one hormone (either TRH/TSH) in response to low T3/T4?

People often say an extra level gives you more control/opportunities for fine-tuning: but do we know what the specifics of the fine-tuning are in this case? What is TRH release sensitive to that TSH isn't or vice-versa?

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    $\begingroup$ "...a lot of energy is wasted in synthesising and releasing a second type of hormone that carries the same information as the first." But they are not the same. This (multiple actors & hormones) is a very common aspect of feedback loops for maintaining homeostasis. Your misperception is akin to saying, "Why does a car have both a gas pedal and a brake? Wouldn't it be more efficient to have only one pedal that moves the car when you press it, and when you don't press it, it doesn't (cause forward movement)?" It would be terribly inefficient/dangerous to have to coast to a stop all the time. $\endgroup$ Oct 8, 2023 at 21:29

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