Both T3 and T4 are studied in hepatocytes. What best describes the action of these thyroid hormones?

• A. Both bind to surface receptors
• B. Both enter the nucleus
• C. T3 is converted to T4
• D. Thyroid receptors prefer T4 over T3

Answer: (B)

Thyroid hormones, specifically T3 (triiodothyronine) and T4 (thyroxine), exert their effects by entering target cells and binding to nuclear receptors, which act as transcription factors that regulate gene expression. This process primarily occurs within the nucleus of the cell, where the hormone-receptor complex influences the transcription of specific genes associated with metabolism, growth, and development.

While both T3 and T4 are capable of entering cells, T3 is significantly more biologically active than T4. However, both hormones undergo conversion within cells, with T4 being converted into T3 by deiodinase enzymes, but this process is not relevant to the main action of the hormones as they relate to nuclear receptor activity.

The statement that both hormones enter the nucleus accurately reflects their mechanism of action, as their nuclear entry leads to alterations in gene expression that drive important physiological changes in hepatocytes and other target tissues. This nuclear interaction underpins the metabolic regulatory role of thyroid hormones in the body.