Feedback Regulation of Hormone Secretion - Anatomy and Physiology

1. Why does dietary iodine deficiency cause the thyroid gland to enlarge (goiter)?

   Without iodine the thyroid cannot make enough T3 and T4, so negative feedback is lost and high TSH keeps stimulating thyroid growth.

2. What are the three types of stimuli that trigger hormone secretion?

   Humoral (changing blood levels of ions or nutrients), neural (nerve fibers), and hormonal (other hormones).

3. What is the difference between long-loop and short-loop feedback in a hypothalamic-pituitary axis?

   Long-loop: the target gland hormone inhibits the hypothalamus and pituitary. Short-loop: a pituitary hormone inhibits the hypothalamus.

4. Which adrenal hormone feeds back to inhibit CRH and ACTH secretion?

   Cortisol.

5. Parathyroid hormone is released when blood calcium falls. Which type of stimulus is this?

   A humoral stimulus, because the gland responds directly to a change in a blood solute.

6. How do rising levels of T3 and T4 affect TRH and TSH secretion?

   They inhibit both: thyroid hormones act on the hypothalamus to reduce TRH and on the anterior pituitary to reduce TSH (negative feedback).

7. The adrenal medulla releases epinephrine when sympathetic preganglionic fibers stimulate it. Which type of stimulus is this?

   A neural stimulus, because nerve fibers directly trigger the secretion.

8. What is the classic example of positive feedback in hormone secretion during the ovarian cycle?

   Sustained high estrogen from the dominant follicle triggers the LH surge, which causes ovulation.

9. In primary hypothyroidism the thyroid gland itself fails. What are the expected TSH and T4 levels?

   High TSH and low T4. Low thyroid hormone removes negative feedback, so the pituitary raises TSH.

10. In Graves disease, antibodies stimulate the TSH receptor. What are the expected TSH and T4 levels?

    High T4 and low TSH. The antibodies drive the thyroid directly, and the excess hormone suppresses pituitary TSH.

11. A patient takes high-dose glucocorticoids for months. What happens to ACTH secretion and the adrenal cortex?

    ACTH is suppressed by negative feedback, so the adrenal cortex atrophies; abrupt withdrawal can cause adrenal insufficiency.

12. In secondary hypothyroidism the anterior pituitary fails. What are the expected TSH and T4 levels?

    Low TSH and low T4, because the thyroid receives too little stimulation.

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