A 39-year-old woman with long-standing untreated primary hypothyroidism had suffered amenorrhea-galactorrhea syndrome and myxoedema. She also had pituitary enlargement with supraseller extention which was found on CT. Her serum T₃ and T₄ were unmeasurably low, and her serum TSH and prolactin were abnormally high. A microsome test gave an abnormally high value. Because she had no visual disturbance, thyroid replacement therapy and follow-up by CT were performed without resort to surgery. Within 2 months after the therapy, pituitary regression was seen on CT. Her serum T₃ and T₄ were normalized. TSH and prolactin levels reduced during the thyroid replacement therapy. Jawadi et al. and Pita et al. reported radiological evidence of pituitary regression or diminution of pituitary tumor by thyroid replacement therapy. These masses might not be a pituitary tumor, but a pituitary hyperplasia caused by the long-standing untreated primary hypothyroidism. A long-standing pituitary hyperplasia may be transformed into a pituitary tumor. The first choice of treatment for this type of pituitary tumor is thyroid replacement therapy unless the patient has a severe visual disturbance. However, if this replacement therapy is not effective for dimination of the tumor, surgical removal of the tumor should be considered.