Answer: Tyrosine residues of thyroglobulin to form T3 and T4.
- A Calcitonin produced by parafollicular cells of the thyroid
- B Tyrosine residues of thyroglobulin to form T3 and T4
- C TSH released by the anterior pituitary gland
- D The receptor protein that binds thyroid hormone in target cells
Correct answer: B. Tyrosine residues of thyroglobulin to form T3 and T4
Explanation: Thyroid follicular cells iodinate tyrosine residues on thyroglobulin, forming triiodothyronine (T3) and tetraiodothyronine/thyroxine (T4); iodine deficiency impairs this synthesis.
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.