Reference intervals for thyroid-stimulating hormone, free thyroxine, and free triiodothyronine in elderly Chinese...

Background Thyroid hormone levels are essential for diagnosing and monitoring thyroid diseases. However, their reference intervals (RIs) in elderly Chinese individuals remain unclear. We aimed to identify factors affecting thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) levels using clinical "big data" to establish hormone level RIs for elderly Chinese individuals. Methods We examined 6781, 6772, and 6524 subjects aged ≥65 years who underwent FT3, FT4, and TSH tests, respectively, at the Peking Union Medical College Hospital between September 1, 2013, and August 31, 2016. Hormones were measured using an automated immunoassay analyzer (ADVIA Centaur XP). RIs were established using the Clinical Laboratory Standards Institute document C28-A3 guidelines. Results The median TSH was significantly higher in women than in men; the opposite was true for median FT3 and FT4 levels. No differences were observed in TSH or FT4 by age in either sex or overall; FT3 levels significantly decreased with age. Seasonal differences were observed in TSH and FT3 levels but not FT4 levels; the median TSH was the highest in winter and lowest in summer, whereas the median FT3 was the lowest in summer (albeit not significantly). RIs for TSH were 0.53-5.24 and 0.335-5.73 mIU/L for men and women, respectively; those for FT3 were 3.76-5.71, 3.60-5.42, and 3.36-5.27 pmol/L in 64- to 74-, 75- to 84-, and 85- to 96-year-old subjects, respectively. The RI for FT4 was 11.70-20.28 pmol/L. Conclusions RIs for TSH in elderly individuals were sex specific, whereas those for FT3 were age specific.