Somatotroph and thyrotroph pituitary cells share a common precursor cell expressing the transcription factor Pitl in ontogeny. Cells expressing both thyrotropin (TSH) and growth‐hormone (GH) are found in adult rat pituitary and in human pituitary adenomas in acromegaly, and these tumors contain both thyrotropin‐releasing hormone (TRH) and the TRH receptors (TRHR). It has been shown that stimulation of TSH expression in primary hypothyroidism promotes changes suggestive of somatotroph to thyrotroph cell transdifferentiation. We tested this hypothesis and the role of TRH in experimental primary hypothyroidism in rats. Adult female Long‐Evans rats, 6 months old, were administered the antithyroid drug methimazole (0,1% w/v) in the drinking water for 42 days. Animals were sacrificed by perfusion fixation under anaesthesia at weekly intervals and pituitary tissue processed in acrylic resin for immunofluorescence and immuno‐electronmicroscopy for TSH, GH and TRHR. In the hypothyroid rat pituitary immunofluorescent somatotrophs were greatly reduced in number and gradually replaced by thyrotrophs during methimazole administration. Colocalization of GH and TSH in the same cell was noted. Immunoelectronmicroscopy demonstrated the development of enlarged thyrotrophs with dilated rough endoplasmic reticulum containing an electron‐dense material and intracisternal granules, both of which are immunoreactive for TSH (‘thyroidectomy cells’). The somatotrophs showed reduced GH immunoreactivity and also the presence of TSH‐type, small‐size secretory granules. This suggests that, the greatly increased number of TSH‐cells in methimazole‐induced‐hypothyroidism is due, at least partially, to the transdifferentiation of somatotroph into thyrotroph cells. TRHR immunofluorescence was expressed in many somatotrophs in normal rat pituitary and unlike immunoreactive GH, its expression was enhanced during hypothyroidism. The number of TRHR‐immunoreactive cells increased in parallel with the number of TSH‐immunoreactive cells. This indicates a role for TRH stimulation in the transdifferentiation process. Taken together, these data suggest that, in addition to the cell mutation mechanism involving an early totipotential progenitor cell, transdifferentiation of existing somatotroph cells also plays a part in the pathogenesis of multihormonal GH‐secreting adenomas.