Which thyroid hormone is considered the inactive form?

• A. T4
• B. T3
• C. MIT
• D. DIT

Answer: (A)

Thyroid hormones differ in how active they are. T3 is the form that actually drives the effects in tissues because it binds receptors and regulates gene expression. T4, on the other hand, acts as a circulating reservoir and prohormone. It’s released by the thyroid and then converted in peripheral tissues (like liver and kidneys) into T3 by deiodinase enzymes, which activates it. Because it itself has relatively low immediate activity, T4 is considered the inactive form in terms of biological action.

MIT and DIT are not circulating hormones themselves; they are the iodinated tyrosine precursors used inside thyroglobulin to build T3 and T4. They’re part of the synthesis process, not active hormone forms.