(Redirected from Astwood-Hoskins-Loop)
Synonyms: Pituitary-thyroid feedback control, pituitary-thyroid axis, thyrotropic axis, Astwood-Hoskins loop
The pituitary gland secretes the glandotropic hormone thyrotropin (TSH) that stimulates secretion of thyroxine (T4) and triiodothyronine (T3) in the thyroid gland. Conversely, thyroid hormones inhibit secretion of TSH in the form of degenerative feedback so that the system achieves equilibrium levels of the involved hormones. Furthermore, TSH-incretion depends from the level of the releasing hormone TRH that is formed in the hypothalamus. TRH thus acts in a way similar to a setpoint of the feedback control.
This main feedback interaction is complemented by additional "plugged in" feedback loops, e.g. an ultra-short-feedback control where TSH inhibits its own secretion, a long-feedback loop connecting thyroid hormones with TRH secretion and feedback loops for plasma protein binding of T4 and T3.
The feedback control's function can be assessed in most cases by determining the following hormones:
In special cases the following parameters are required:
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