Which statement best describes the negative feedback regulation of thyroid hormone production?

• A. Free T4 and Free T3 provide negative feedback to the hypothalamus and pituitary.
• B. TSH provides negative feedback to the hypothalamus.
• C. TRH provides negative feedback to the pituitary.
• D. Calcitonin primarily regulates calcium balance.

Answer: (A)

Thyroid hormone production is regulated by negative feedback from the circulating hormones themselves. When free T4 and free T3 levels are high, they signal both the hypothalamus and the pituitary to dial back their activity: the hypothalamus reduces TRH release, and the pituitary reduces TSH release. With less TSH, the thyroid gland makes less T4 and T3, helping bring levels back toward normal. This keeps thyroid hormone levels steady through a balanced loop.

Calcitonin, while a thyroid-derived hormone, mainly influences calcium balance and does not regulate thyroid hormone production. TRH acts as a stimulant to the pituitary to release TSH rather than providing negative feedback, and TSH primarily drives the thyroid to make thyroid hormones rather than feeding back to suppress the hypothalamus directly.