We report a case of reversible pituitary enlargement due to hypothyroidism in a 3-year-old male. OnT1-weighted images, the mass was located in the anterior lobe of the pituitary gland, and the signal intensity ofthe mass was equal to that of brain cortex. Gd-DTPA enhanced T1-weighted images showed homogeneous enhancement ofthe mass. After supplemental therapy with thyroid hormone for 8 months, MRI showed that the pituitary gland wasmarkedly smaller.