Physiology Questions

1 & 2

The hypothalamus secretes TRH into the hypothalamohypophysial portal system, which stimulates the anterior pituitary to secrete TSH. TSH in turn causes the thyroid gland to release thyroxine. Thyroxine and other thyroid hormones affect metabolic rate.

Among thyroxine’s many target organs are the hypothalamus and the anterior pituitary. Thyroxine acts on these organs to inhibit their secretion of TRH and TSH (thyroid-stimulating hormone), respectively. This negative feedback inhibition is essential for homeostasis because it keeps the thyroxine levels fairly constant.

The hormone thyroxine contains the element iodine; without iodine, the thyroid gland cannot produce thyroxine. A lack of thyroxine reduces inhibition and reduced inhibition results in elevated secretion of TRH and TSH. High levels of TSH stimulate the thyroid gland, whose cells enlarge in a futile attempt to manufacture more thyroxine, but they cannot without iodine. This results in a goitre, which is an enlarged thyroid gland.

In terms of growth:

In tadpoles, for instance, at the premetamorphic stage, the hypothalamus stimulates the adenohypophysis to secrete TSH. TSH then stimulates the thyroid gland to secrete thyroxine. Thyroxine binds to its receptor and initiates the changes in gene expression necessary for metamorphosis. As metamorphosis proceeds, thyroxine reaches its maximal level, after which the forelimbs begin to form and the tail is reabsorbed.

Some of the most dramatic effects of thyroid hormones are observed in their regulation of growth and
development. In developing humans, for example, thyroid hormones promote growth of neurons and stimulate maturation of the central nervous system. Children born with hypothyroidism are stunted in their growth and suffer severe mental retardation, a condition called cretinism. Early detection through measurement of thyroid hormone levels allows this condition to be treated with thyroid hormone administration.

Thyroid hormones often function cooperatively, or synergistically, with other hormones, promoting the activity of growth hormone, epinephrine, and reproductive steroids. Through these actions, thyroid hormones function to ensure that adequate cellular energy is available to support metabolically demanding activities.

3. I've always know propylthiouracil as a drug used to manage hyperthyroidism which is caused by an overactive thyroid gland.

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