Answer: Iodine.
- A Iron
- B Zinc
- C Calcium
- D Iodine
Correct answer: D. Iodine
Explanation: Iodine is essential for synthesising thyroid hormones (T<sub>3</sub> and T<sub>4</sub>); its deficiency can cause goitre.
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.