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

• A. Euthyroid with low TBG (nephrotic syndrome, cirrhosis)
• B. Subclinical hypothyroidism
• C. Subclinical hyperthyroidism
• D. Primary hyperthyroidism

Answer: (A)

Think about how thyroid tests reflect hormone levels and binding proteins. TSH and FT4 measure the actual circulating free hormones and pituitary response, while TT3 and TT4 reflect the total amount of hormone in the blood, which depends on how much thyroid-binding globulin (TBG) is present. If TBG is reduced, less hormone is bound, so total T3 and total T4 fall, but the free (biologically active) hormones stay normal because the body maintains them. The T3 uptake test shifts upward when there are fewer binding sites on TBG, so it increases in low-TBG states.

This pattern—normal TSH, normal FT4, low TT3/TT4, and high T3 uptake—fits a euthyroid person with reduced TBG, as seen in conditions like nephrotic syndrome or cirrhosis. In true thyroid disease, FT4 or TSH would be abnormal, so the same profile wouldn’t occur.