In patients with protracted primary hypothyroidism, the pituitary is enlarged due to the lack of feedback inhibition by thyroid hormone. In the present work, adenohypophysial biopsies from three women with protracted primary hypothyroidism were investigated by routine histology, immunocytochemistry, double immunostaining, immunoelectron microscopy, and combined immunocytochemistry – in situ hybridization. These methods confirmed the presence of massive thyrotroph hyperplasia and the formation of ”thyroidectomy” or ”thyroid deficiency” cells. A number of thyroidectomy cells were found to be immunoreactive for growth hormone (GH). Double immunostaining and immunoelectron microscopy revealed the presence of bihormonal cells containing both GH and thyroid stimulating hormone (TSH). Immunostaining combined with in situ hybridization revealed GH immunoreactive cells expressing TSH mRNA as well as TSH immunopositive cells expressing GH mRNA. Our findings provide conclusive evidence that somatotrophs may transform to thyrotrophs. Thus, in addition to multiplication of thyrotrophs, transdifferentiation of GH cells to thyrotrophs contributes to the increase of TSH-producing cells. The presence of such bihormonal cells best termed ”thyrosomatotrophs” supports the concept that adenohypophysial cells are not irreversibly committed to the production of one single hormone and that their phenotype can change in response to functional demand.