We present the case of a 29-year-old woman who developed a severe hypothyroidism induced by a thyroxine malabsorption and a secondary pituitary hyperplasia. We performed thyroxine absorption tests to diagnose the malabsorption and to evaluate the best therapeutic intervention. Once assessed a correct therapy lowering TSH, we observed the regression of pituitary mass confirming our diagnosis of secondary pituitary hyperplasia. We suggest to evaluate any possible reason for thyroxine malabsorption and to consider the hypothesis of pituitary hyperplasia in the presence of pituitary mass together with overt hypothyroidism.