Final answer:
Thyroid-Stimulating Hormone (TSH) regulates the thyroid gland and is secreted by the anterior pituitary in response to TRH from the hypothalamus. The negative feedback loop involving TSH, T3, and T4 maintains thyroid hormone levels within a narrow range.
Step-by-step explanation:
Thyroid-Stimulating Hormone (TSH)
The function of Thyroid-Stimulating Hormone (TSH) is to regulate the activity of the thyroid gland. TSH, also known as thyrotropin, is secreted by the anterior pituitary gland in response to thyrotropin-releasing hormone (TRH) from the hypothalamus. The target organ of TSH is the thyroid gland, where it stimulates the release of thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).
Negative Feedback System
The secretion of TSH is controlled by a negative feedback loop involving the hypothalamus, pituitary gland, and thyroid gland. When the levels of T3 and T4 increase in the blood, they exert negative feedback on the hypothalamus and anterior pituitary to inhibit further production of TSH. This tight regulation ensures that thyroid hormone levels remain within a narrow optimal range, preventing either deficiency or excess.
TRH is the hypothalamic releasing factor that stimulates the pituitary to secrete TSH. The system represents an elegant example of hormonal control through negative feedback, where the end product (T3 and T4) inhibits the stimulus (TRH and TSH) that initiated its production, maintaining homeostasis.