Euthyroid with low TBG occurs in which situations?

• A. Nephrotic syndrome, cirrhosis
• B. Estrogen therapy
• C. Pregnancy
• D. Inflammation

Answer: (A)

The main idea being tested is how thyroid-binding proteins affect measured thyroid hormones. Thyroxine and triiodothyronine in the blood are mostly bound to proteins, especially thyroid-binding globulin (TBG). The free (unbound) hormone is the biologically active form. If TBG levels fall, total T4 and T3 decrease because there’s less binding protein to carry them, but the free hormone remains normal, so the person is clinically euthyroid.

This happens notably in nephrotic syndrome, where large amounts of protein are lost in the urine, including TBG, and in cirrhosis, where liver capacity to synthesize TBG is reduced. In both cases, you’d expect low total thyroid hormone levels with a normal free fraction.

Estrogen therapy and pregnancy raise TBG levels because estrogen stimulates hepatic production of binding proteins, so total T4/T3 can appear elevated even though the free hormone is normal. Inflammation can lower TBG somewhat as an acute-phase reaction, but the classic, textbook scenarios showing low TBG with preserved free hormone are nephrotic syndrome and cirrhosis.