Question

Which of the following is an active form of thyroid hormone that acts through a nuclear receptor to stimulate energy-yielding metabolism by increasing the expression of genes encoding key catabolic enzymes?

• A. Thyroglobulin (Tyr) \
• B. Thyroglobulin (Tyr-I) \
• C. Thyroxine (T4) \
• D. Triiodothyronine (T3)

Hint:

$\text{T}_{4}$ is the long-lasting form shipped through the blood, but $\text{T}_{3}$ is the high-powered active form that actually turns on metabolic genes inside the nucleus.

Answer 1

The Correct Option is D

Step 1: Concept The thyroid gland secretes iodothyronine hormones that regulate the body's basal metabolic rate. These hormones travel to target cells, enter the nucleus, and bind to specific nuclear receptors to alter gene transcription.

Step 2: Meaning While the thyroid gland releases more thyroxine ($\text{T}_{4}$), it acts primarily as a prohormone. It must be converted into the more active form by target tissues to trigger maximum metabolic activity.

Step 3: Analysis * Thyroglobulin (A and B) is a large precursor protein stored inside the thyroid follicles, not a free active hormone. * Thyroxine ($\text{T}_{4}$) (C) is the main form secreted by the thyroid gland, but it binds to nuclear thyroid hormone receptors with low affinity. * Triiodothyronine ($\text{T}_{3}$) (D) is produced when $\text{T}_{4}$ is deiodinated in target tissues. $\text{T}_{3}$ binds to nuclear thyroid receptors with a 10-fold higher affinity than $\text{T}_{4}$. This makes it the primary active form that drives the transcription of metabolic genes.

Step 4: Conclusion Thus, Triiodothyronine ($\text{T}_{3}$) is the primary active form of the hormone that triggers cellular catabolism via nuclear receptors, matching option D. Final Answer: (D)