Pituitary neoplasms in primary myxedema are rare and previously reported cases were described as chromophobe adenomas. None of them were subjected to modern methods of study. A 26 yr old woman with untreated primary hypothyroidism developed a pituitary tumor which produced bitemporal hemianopsia. Histochemical and ultrastructural studies characterized the tumor cells as thyrotrophs. The secretory granules present had a mean diameter of 110 .mu.m and were membrane-bound and surrounded by clear halos. Many of the tumor cells were actively secreting. They had reduced numbers of granules, possessed expanded Golgi complexes, abundant rough endoplasmic reticulum and free ribosomes. Cytoplasmic microfilaments were also seen. A possible mechanism of production of this tumor is discussed.