The body is able to maintain a relatively constant level of thyroid hormone in the blood because
a. Thyroid hormone stimulates the pituitary to secrete thyroid-stimulating hormone (TSH).
b. Thyroid hormone inhibits the secretion of TSH-releasing hormone (TRH) from the hypothalamus.
c. TRH inhibits the secretion of thyroid hormone by the thyroid gland.
d. Thyroid hormone stimulates the hypothalamus to secrete TRH.

Verified step by step guidance

1. Understand the concept of negative feedback in endocrine systems: Negative feedback is a regulatory mechanism in which a change in a physiological variable triggers a response that counteracts the initial change, helping to maintain homeostasis.
2. Identify the key players in the regulation of thyroid hormone levels: The hypothalamus secretes thyroid-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce thyroid hormones (T3 and T4).
3. Recognize the role of thyroid hormone in feedback regulation: When thyroid hormone levels in the blood are sufficient, they inhibit the secretion of TRH from the hypothalamus and TSH from the pituitary gland. This prevents overproduction of thyroid hormones, maintaining a stable level.
4. Analyze the options provided: Option (a) suggests a positive feedback loop, which is incorrect because thyroid hormone does not stimulate TSH secretion. Option (b) aligns with the concept of negative feedback, where thyroid hormone inhibits TRH secretion. Option (c) incorrectly states that TRH inhibits thyroid hormone secretion, which is not true. Option (d) incorrectly suggests that thyroid hormone stimulates TRH secretion, which contradicts the negative feedback mechanism.
5. Conclude that the correct answer is based on the negative feedback mechanism, where thyroid hormone inhibits the secretion of TRH from the hypothalamus, as described in option (b).

Key Concepts

Thyroid Hormone Regulation

Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), play a crucial role in regulating metabolism and energy levels in the body. Their levels are maintained through a feedback loop involving the hypothalamus and pituitary gland, ensuring that hormone production is adjusted based on the body's needs.

Hypothalamic-Pituitary-Thyroid Axis

The hypothalamic-pituitary-thyroid (HPT) axis is a complex set of interactions between the hypothalamus, pituitary gland, and thyroid gland. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary to secrete thyroid-stimulating hormone (TSH), prompting the thyroid to produce thyroid hormones. This axis is essential for maintaining hormonal balance.

Negative Feedback Mechanism

Negative feedback is a regulatory mechanism in biological systems where an increase in a substance leads to a decrease in its production. In the context of thyroid hormones, elevated levels of T3 and T4 inhibit the release of TRH and TSH, thereby reducing further thyroid hormone production, which helps maintain homeostasis.