Functioning oxyphil parathyroid adenoma.

A hormonally active oxyphil adenoma of the parathyroid masqueraded as a thyroid nodule and was present as a palpable neck mass for more than six years prior to the onset of clinically obvious hyperparathyroidism. The slow evolution of the hyperparathyroidism in the presence of a relatively large adenoma was correlated with the presumptive low hormonal synthetic capacity of oxyphil cells that formed the bulk of the tumor.