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A hormone cascade pathway on the example of thyroid regulation

A hormone cascade pathway is often the result of interactions between the hypothalamus, anterior pituitary, and the target endocrine gland.  In response to the signals from the brain, the hypothalamus secretes neurohormones that effect (stimulate or inhibit) the anterior pituitary gland, which in turns secretes hormones that act on the target cells. Note that hypothalamic neurosecretory cells are capable of converting neural signals into hormonal output, and are thus called neuroendocrine transductors. Such transductors allow the nervous system to control the endocrine system and as a consequence, allow hormones to respond to environmental and psychological stimuli.

Thyroid regulation

Thyrotropin RH -> Thyroid SH (thyrotropin) -> T3, T4

Various environmental factors, like cold and acute stress, can stimulate the release of thyrotropin-releasing hormone (TRH), or thyroid stimulating hormone releasing hormone (TSH-RH) from the hypothalamus.  Once released, THR stimulates the thyrotroph cells in the anterior pituitary gland to secrete thyroid-stimulating hormone (TSH), or thyrotropin. TSH stimulated the thyroid gland to release thyroid hormone, which is a generic name for a pair of two hormones, triiodothryronine(T3)  thyroxine(T4). T4 is much more (98%) prevalent in blood that T3 (2%).

Hormone cascade pathways usually involve the mechanism of negative feedback. In the above-mentioned example, the thyroid hormone itself carries the negative feedback by inhibiting TSH release from the anterior pituitary gland, and TRH from the hypothalamus. This way, the hormone cascade pathway is a self limiting mechanisms, which prevents overproduction in the thyroid gland.

Functions of the thyroid hormones in humans:

  • Regulate bioenergetics
  • Help maintain normal blood pressure
  • Help maintain normal heart rate
  • Help maintain muscle tone
  • Regulate digestive functions
  • Regulate reproductive functions
  • Nerve cells branching and bone-forming in embryonic development