Thyroid disease mainly has a thyroid origin but can occasionally have a pituitary origin. Clinicians face several challenges when these conditions occur together. We aimed to determine the prevalence of thyroid disorders in patients undergoing transsphenoidal adenomectomy (TSA) for pituitary disease. We reviewed the medical records of patients undergoing TSA for pituitary disease between 2008 and 2017 at Severance Hospital. Thyroid disorders were categorized using blood test results and medical history at the time of preoperative evaluation. Among 2202 patients, 44 (2.0%), 218 (9.9%), and 74 (3.4%) had hyperthyroidism, hypothyroidism, and post-thyroidectomy status before TSA, respectively. Among the 44 patients with hyperthyroidism, 30 (68.2%) had secondary hyperthyroidism. Among the 218 patients with hypothyroidism, 165 (75.7%) had secondary hypothyroidism. Secondary hypothyroidism was more common in patients with adrenocorticotropic hormone-secreting pituitary adenoma (adjusted odds ratio [aOR] 1.85), Rathke’s cleft cysts (aOR 2.34), and craniopharyngioma (aOR 2.58) (all p<0.05) than in those with nonfunctioning pituitary adenoma. Contrastingly, thyroid cancer showed increased prevalence in patients with growth hormone- (aOR 3.17), prolactin- (aOR 3.66), and thyroid-stimulating hormone-secreting (aOR 6.28) pituitary adenomas (all p<0.05). Pituitary disease sometimes accompanies thyroid disorders; their characteristics vary according to the type of pituitary disease.