Central Hypothyroidism with Low TSH Compared to Normal TSH Is Associated with More Advanced Pituitary Disease and Less Favorable Metabolic Profile

<b>Background:</b> Central hypothyroidism is characterized by either decreased TSH or, more commonly, normal TSH. This study aims to check whether this biochemical difference related to the severity of the pituitary disease, metabolic processes and general well-being. <b>Methods:</b> A retrospective analysis was performed on 108 inpatients with hypopituitarism, aged 18–80, hospitalized (1 January 2020, through 31 December 2022) in the Department of Endocrinology and Metabolic Diseases, Medical University of Lodz, Poland. Hypopituitary patients with central hypothyroidism (<i>n</i> = 90) were divided into two subgroups: patients with TSH below normal ranges (low TSH; <i>n</i> = 52) and patients with TSH in reference ranges (normal TSH; <i>n</i> = 38). <b>Results:</b> Among patients with central hypothyroidism, surgical treatment due to pituitary disease was performed more commonly in those with low TSH than in those with normal TSH (65 vs. 42%, <i>p</i> = 0.010). Expectedly, five pituitary deficiencies were diagnosed more commonly in patients with low TSH than in those with normal TSH (46 vs. 13%, <i>p</i> = 0.001). In a regression analysis, the ACTH concentration was the only independent determinant negatively associated with low TSH (also after limiting the analysis to non-treated patients). Regarding lipid profile, decreased HDL cholesterol occurred more commonly in patients with low TSH vs. normal TSH (44% vs. 23%; <i>p</i> = 0.033), which was also observed after the limitation to non-treated patients (47% vs. 21%; <i>p</i> = 0.013). <b>Conclusions:</b> Low TSH in patients with central hypothyroidism is associated with more advanced pituitary disease and less favorable metabolic profile.