Regulation of Thyroid Hormone Secretion: Hormonal Pathways and Feedback

Q: Identify the hormones labeled A–C in the figure below showing the regulation of thyroid hormone secretion. Refer to Figure 16.7 for help.

Background

Topic: Endocrine Regulation and Hormonal Pathways

This question tests your understanding of the hormonal control of thyroid hormone secretion, specifically the hypothalamic-pituitary-thyroid axis. It also involves recognizing the feedback mechanisms that regulate hormone levels.

Key Terms and Formulas

• Hypothalamus: Releases hormones that regulate the pituitary gland.

• Anterior Pituitary: Releases tropic hormones that stimulate other endocrine glands.

• Thyroid Gland: Produces thyroid hormones (T3 and T4).

• TRH: Thyrotropin-releasing hormone (from hypothalamus)

• TSH: Thyroid-stimulating hormone (from anterior pituitary)

• T3/T4: Triiodothyronine and thyroxine (from thyroid gland)

• Negative Feedback: A process where increased hormone levels inhibit further hormone release.

Step-by-Step Guidance

1. Examine the diagram: The arrows show the flow of hormonal signals from the hypothalamus to the anterior pituitary, then to the thyroid gland, and finally to target cells.

2. Recall the sequence of hormone release: The hypothalamus releases a hormone (A) that stimulates the anterior pituitary.

3. The anterior pituitary releases a hormone (B) that stimulates the thyroid gland.

4. The thyroid gland releases hormones (C) that act on target cells throughout the body.

5. Consider the feedback loops: The hormones released by the thyroid gland can inhibit the release of hormones from both the hypothalamus and anterior pituitary, as shown by the dashed lines.

Try solving on your own before revealing the answer!

Final Answer:

A: TRH (Thyrotropin-releasing hormone)

B: TSH (Thyroid-stimulating hormone)

C: T3/T4 (Triiodothyronine and Thyroxine)

These hormones are released sequentially, and the thyroid hormones (T3/T4) provide negative feedback to both the hypothalamus and anterior pituitary to regulate their own production.