Residual pituitary enlargement in primary hypothyroidism despite 1 1/2 years of L-thyroxine therapy.

Pituitary hyperplasia consequent to untreated primary hypothyroidism undergoes impressive regression following thyroxine therapy. We describe a young man with primary hypothyroidism and pituitary hyperplasia, who showed significant but incomplete regression of pituitary enlargement despite 18 months of L-thyroxine therapy (1 year in replacement dose (150 micrograms/day) and an additional 6 months in suppressive dose (300 micrograms/day)).