Aim: Thyroid hormone tests show circadian rhythm. In this study we investigated the changes of Thyroid Stimulating Hormone (TSH) and free Thyroxine (fT4) test results in apparently healthy individuals and thyroid hormone replacement patients at different times of the day.

Material and Methods: The study included apparently healthy volunteers (n=10) and patients who received replacement therapy (n=10) with the diagnosis of hypothyroidism. Blood samples were taken from each patient three times in the same day. Samples are named based on phlebotomy time, Group A samples were collected between 8.00-9.00 am, Group B samples were collected between 11.00-12.00 am and Group C samples were collected between 15.00-16.00 pm. Serum TSH and fT4 levels were measured by immunoassay method in the Roche Hitachi e170.

Results: The change of TSH test results were found 29,9% between Group A-B, 32,2% between Group A-C and 10,6% between Group B-C in apparently healthy volunteers. The change of fT4 test results were found below 10% in all groups in apparently healthy volunteers. The change of TSH test results were found 27,4% between Group A-B, 30,2% between Group A-C, 10,1% between Group B-C in thyroid hormone replacement patients. The change of fT4 was %13 between Group A-B and below 10% in other groups in thyroid hormone replacement patients.

Conclusion: TSH which is the most commonly used test for the diagnosis and treatment of thyroid diseases, shows circadian rhythm during the day. Thus, the phlebotomy time of the sample is very important. In our study the results of the TSH test were higher in 8.00-9.00 am whereas fT4 results did not change clinically significant between the hours.