Background: Reference ranges for serum thyrotropin (TSH), free thyroxine (fT4), and free triiodothyronine (fT3) established without considering age- and sex-based differences are currently used to evaluate thyroid function. Therefore, we investigated age- and sex-based differences in serum TSH and thyroid hormone levels in euthyroid individuals. Methods: We performed cross-sectional analyses of retrospective data collected from two Japanese institutions. We estimated sex-specific 95% reference ranges for TSH and fT4 according to age strata. Results: We included data from 14,860 participants undergoing screening with a Siemens thyroid testing kit and 8,132 participants undergoing screening with an Abbott kit during annual health check-ups at Takasaki Hidaka Hospital. In addition, 515 participants visiting a specialized thyroid-focused hospital were evaluated using Tosoh kits. The median TSH level of women in their 30s was 1.5 mIU/L (2.5th percentile, 0.5; 97.5th percentile, 4.6) using the Siemens kit, while that of women in their 60s was 1.9 (0.7-7.8) mIU/L. The corresponding levels were lower in men; the age-associated increase was small. The median serum fT4 level of men in their 30s was 1.3 (1.0-1.7) ng/dL and that of men in their 60s was 1.2 (1.0-1.6) ng/dL. These levels gradually but significantly decreased with age. fT4 levels in women were lower than those in men and remained consistent with age. Serum fT3 levels were significantly higher in men than in women and gradually but significantly decreased with age. The Abbott and Tosoh kits showed similar results. When using the Siemens kit, ∼60% (216/358) of women diagnosed with subclinical hypothyroidism using manufacturer-recommended reference ranges had normal results when age- and sex-specific reference ranges were applied, demonstrating the high percentage of overdiagnosis, especially in those aged ≥60 years. Conversely, some middle-aged individuals with normal thyroid function were reassessed and classified as having subclinical hyperthyroidism by age- and sex-specific reference ranges. Conclusions: Age- and sex-specific reference ranges should be used to avoid over- and underdiagnosis of subclinical thyroid dysfunction and appropriate therapies.

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