Pituitary thyrotroph hyperplasia secondary to primary hypothyroidism is a rare cause of pituitary enlargement. We report a case of pituitary hyperplasia in a young woman thought to have a pituitary macroadenoma with suprasellar extension. A 21-year-old woman was admitted to our hospital, complaining of menstrual irregularity and galactorrhea. Serum thyrotropin and prolactin were high, and serum-free thyroxine was low. Magnetic resonance imaging revealed symmetrical pituitary enlargement associated with contrast enhancement simulating macroadenoma. After adequate hormone replacement therapy with levothyroxine, both symptoms and pituitary hyperplasia regressed within 4 months. It is concluded that pituitary tumor associated with elevated thyroid stimulating hormone and low levels of free T4 should always be treated with replacement thyroxine before other diagnostic tests are ordered.