Question

Assertion (A): Thyroxine (T4) is a more important physiological hormone than triiodothyronine (T3). Reason (R): T4 acts more slowly than T3.

• A. Both A and R are individually true and R is the correct explanation of A
• B. Both A and R are individually true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Compare which hormone binds the nuclear receptor with higher affinity, and which one is converted from the other.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question tests whether T4 or T3 is the physiologically more important thyroid hormone, and whether T4 truly acts more slowly than T3.

Step 2: Key Concept or Approach:
T4 is secreted in far greater amount by the thyroid gland, about ninety percent of total output, but it mainly functions as a circulating reservoir or prohormone. It is converted peripherally by 5'-deiodinase into T3, and it is T3 that binds the nuclear thyroid hormone receptor with much higher affinity and drives most of the metabolic and physiological effects. So T3, not T4, is the more biologically active and physiologically important hormone, even though T4 circulates at a higher concentration.

Step 3: Working Through the Options:
A is false because it reverses the true relationship, T3 is the more important, more potent hormone at the receptor level, and T4 mainly serves as its precursor. R, however, is true on its own, T4 does act more slowly than T3 because it has a longer half-life of around seven days compared with roughly one day for T3, and it must first be converted to T3 peripherally before it can exert its full effect, so its onset and offset of action are both slower.

Step 4: Conclusion:
A is false while R is true, matching the option stating A is false but R is true.