Which condition shows TSH normal, FT4 normal, TT3/TT4 high, and T3 uptake low due to high thyroid-binding globulin?

• A. Euthyroid with high TBG (pregnancy, estrogen, inflammation)
• B. Primary hypothyroidism
• C. Subclinical hyperthyroidism
• D. Secondary hyperthyroidism

Answer: (A)

This item tests how binding proteins influence thyroid hormone measurements. When thyroid-binding globulin (TBG) levels are high, more thyroid hormone in the blood is bound, so total T3 and T4 rise even though the free (active) hormones remain normal. Because the regulatory axis cares about free hormone, TSH stays normal in this euthyroid state. The T3 uptake test reflects how much unbound binding sites are available; with high TBG, there are fewer available sites for the assay, so the uptake value appears low. This combination—normal TSH, normal free T4, elevated total T3 and T4, and low T3 uptake due to high TBG—fits a euthyroid patient with increased TBG, as seen in pregnancy, estrogen use, or inflammatory states. Other patterns would involve abnormal TSH or abnormal free hormone levels, which aren’t present here.