This is a 43 years old woman, complaining of galactorhea, headache and hyperprolactinemia. A large macroadenoma of the pituitary with extensive suprasellar extension and displacement of the optic chiasm was detected on MRI and primary hypothyroidism was found. After treatment with levothyroxine for four months, her TSH and prolactin levels returned to normal, her symptoms improved and a marked shrinkage of the selar lesion was observed. The visual fields returned to normal. This is a case of pituitary hyperplasia due to primary hypothyroidism.