Thyroid hormone levels among adults in India: A cross-sectional study

Age and sex influence thyroid physiology, but India-specific, age- and sex-stratified distributions for thyroid-stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) remain variably defined. Hence, we conducted a multicentre, community-based cross-sectional study across five Indian regions, enrolling euthyroid adults without known thyroid disease. Morning fasting samples were assayed on standardized chemiluminescent platforms; nonparametric percentiles were used to derive reference limits and regression models quantified associations. Among 5,812 adults (52.4% women; median age 41 years), median TSH was higher in women than men and increased with age, while FT4/FT3 declined modestly with age. The 97.5th percentile for TSH rose from ~4.3-4.6 mIU/L in 18-29 year-olds to ~6.1-6.4 mIU/L in those ≥60 years, with consistently higher upper limits in women. Thus, we show age- and sex-specific interpretation of thyroid tests in Indian adults.