Effect of sampling time on estimates of thyroid-stimulating hormone, free thyroxine, and free triiodothyronine levels.

Despite the need for precise measurement of thyroid hormones levels for disease detection and monitoring, the importance of sampling time for the accuracy of widely used assays is unclear. In this study, the effects of the time of day on thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) levels were retrospectively evaluated using data obtained from the Laboratory Information System of Peking Union Medical College Hospital. TSH concentrations were significantly lower, and FT4 and FT3 concentrations were higher in males than in females (all p < .05). TSH decreased from 7 am to 1 pm and increased from 1 pm to 4 pm. FT3 concentrations differed among sampling time groups (p < .05). Compared to those at 7 am, the deviations in TSH, FT4, and FT3 were -24.53% to -7.41%, -0.84% to 0.84%, and -0.66% to 0.33%, respectively. TSH levels fluctuated during the day but FT4, FT3 did not exhibit a circadian rhythm. The time of sample collection must be standardized for TSH measurements.