Answer: Low T 3 /T 4 reduces negative feedback on the pituitary, so rising TSH chronically stimulates the thyroid to grow.
- A Excess calcitonin from parafollicular cells causes the gland to hypertrophy according to most studies in the majority of documented cases
- B Autoimmune destruction of the gland triggers compensatory follicular cell division as widely reported in standard reference material
- C High T<sub>3</sub>/T<sub>4</sub> directly stimulates follicular cells to enlarge, independent of any pituitary signal under usual circumstances
- D Low T<sub>3</sub>/T<sub>4</sub> reduces negative feedback on the pituitary, so rising TSH chronically stimulates the thyroid to grow
Correct answer: D. Low T<sub>3</sub>/T<sub>4</sub> reduces negative feedback on the pituitary, so rising TSH chronically stimulates the thyroid to grow
Explanation: Iodine deficiency lowers T<sub>3</sub>/T<sub>4</sub> synthesis, removing negative feedback on the pituitary, so chronically elevated TSH overstimulates the thyroid follicular cells, enlarging the gland.
The hypothalamus releases hormones that direct the pituitary, which releases its own trophic hormone to a target gland; that gland's hormone output then feeds back (dashed lines) to suppress both the pituitary and hypothalamus once levels are sufficient - a self-correcting loop running the entire endocrine system.
Concept context
The endocrine glands, hormone classification and mechanism of action, and negative feedback regulation.