Objective: Thyroid function tests (TFT) are one of the most frequently requested biochemical parameters by clinicians and can be requested at any time of the day. For this reason, we evaluated the effects of diurnal variation and fasting-satiety status on TFT in order to make an accurate and reliable decision in the diagnosis and follow-up of patients.

Material and Methods: Blood samples were taken from 16 healthy adult volunteers (11 males, 5 females) at 09.00, 10.00, 11.00, 12.00, 15.00, 18.00 and 24.00 hours. The samples taken at 09.00 were accepted as basal. Thyroid stimulating hormone (TSH), free triiodothyronine (fT3) and free thyroxine (fT4) levels at other hours were compared with basal levels statistically and clinically.

Results: There was no clinically significant difference in fT4 concentrations during the day. TSH had a variation up to (-27.37%)-(48.95%) within the day. The variations in fT3 (-5.59%) only at 15.00 and 18.00 were found clinically significant.

Conclusion: When evaluating TSH and fT3 test results, it was thought that diurnal variation should be taken into account, and if not, blood sampling should be done at the same time in the morning.