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

Background: The timing of sample collection is crucial for analytes with a circadian rhythm. Although blood samples are usually collected in the morning, they can also be collected in the afternoon. Our study aims to evaluate the impact of sampling time on Thyroid-Stimulating Hormone (TSH), free thyroxine (fT4), and free triiodothyronine (fT3).

Methods: Serum TSH, fT4, and fT3 measurements from 216,278 patients who visited our institute's outpatient clinics over two years (2021-2023) were obtained from laboratory data. After exclusions, 5,953 patient results collected from 8 am to 5 pm were grouped into 1-hour intervals. All measurements were performed using the Cobas e801 (Roche Diagnostics, Penzberg, Germany) with the electrochemiluminescence immunoassay method. Statistical significance was tested with the standard normal deviation test (Z-test), and clinical significance was assessed using bias % values obtained from biologic variation estimates on the website of the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM).