Thyrotropin releasing hormone functions to:
A. Decrease release of T4.
B. Convert T4 to T3.
C. Increase release of thyroid stimulating hormone.
D. Couple iodine to thyroglobulin.
The correct answer and explanation is:
Correct Answer: C. Increase release of thyroid stimulating hormone.
Explanation:
Thyrotropin-releasing hormone (TRH) is a tripeptide hormone secreted by the hypothalamus. Its main role is to regulate the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary gland. TRH travels through the hypothalamic-hypophyseal portal system to reach the anterior pituitary, where it binds to receptors on thyrotroph cells and stimulates the release of TSH into the bloodstream.
TSH then acts on the thyroid gland, promoting the synthesis and secretion of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are crucial for regulating metabolism, growth, and development. T3 is the more biologically active form, and while T4 is secreted in larger quantities, much of it is later converted to T3 in peripheral tissues like the liver and kidneys.
The release of TRH is controlled through a negative feedback loop. When circulating levels of T3 and T4 are sufficient, they inhibit the secretion of both TRH from the hypothalamus and TSH from the pituitary. If thyroid hormone levels drop, TRH secretion increases to stimulate more TSH and, in turn, more T3 and T4.
The other options are incorrect for the following reasons:
- A. Decrease release of T4 is wrong because TRH initiates a cascade that increases T4.
- B. Convert T4 to T3 is incorrect because this conversion occurs in peripheral tissues and is not the function of TRH.
- D. Couple iodine to thyroglobulin is incorrect as this is a function carried out by thyroid peroxidase in the thyroid gland, not by TRH.
Thus, the function of thyrotropin-releasing hormone is to increase the release of thyroid-stimulating hormone (TSH) from the anterior pituitary gland.