Which statements regarding thyroid hormones are not correct?

• A. II, III
• B. III, IV
• C. I, IV
• D. I, II

Answer: (D)

Thyroid hormones regulate metabolism and development through two main hormones, T3 and T4. T3 is the more active form, while much of the circulating T3 comes from peripheral conversion of T4 in tissues. They work by binding to thyroid hormone receptors in the cell nucleus, changing gene transcription, which increases the basal metabolic rate, heat production, and affects heart function, lipid and carbohydrate metabolism, and growth. Synthesis requires iodine and is regulated by the hypothalamic-pituitary-thyroid axis with negative feedback: when levels are enough, TRH and TSH release decreases, reducing production.

Two statements about thyroid hormones that would be incorrect are those that misidentify the active form or the action mechanism—such as claiming T4 is the primary active hormone or that thyroid hormones act without entering cells or without nuclear receptors. The correct understanding aligns with T3 being the active form, T4 serving largely as a circulating prohormone, and the hormones exerting genomic effects via nuclear receptors, with production and release governed by negative feedback.