A 41-year-old male with primary hypothyroidism and a huge TSH-secreting pituitary tumor presented with a bleeding nasal mass that was initially misdiagnosed as a paraganglioma. Other unique features of the case include lack of complaints related to hypothyroidism, an extremely elevated TSH level of 3474 mU/l, and a low prolactin level. The presence of primary hypothyroidism made differentiating TSH-secreting pituitary adenoma from secondary thyrotroph hyperplasia difficult. A low molar ratio of alpha-subunit to TSH on presentation, together with normalization of TSH level and a 50% reduction in the size of the tumor after 6 weeks of thyroxine replacement therapy, suggested the presence of thyrotroph hyperplasia. However, the lack of further decrease in the size of the tumor that was associated with increased metabolic activity on 18-FDG PET scan, intense uptake on octreotide scan, and an elevated alpha-subunit to TSH molar ratio despite the normalization of free T4 and TSH levels for 16 months suggested the coexistence of thyrotroph adenoma. Together, the findings support the view that thyrotroph adenoma/irreversible hyperplasia can result from long standing primary hypothyroidism.