Study on diurnal variation in TSH and freeT4 levels of healthy adults

Thyroid stimulating hormone (TSH) secretion shows a circadian rhythm, affecting free thyroxine (fT4) secretion in the same manner. Samples are collected randomly throughout the day for serum TSH & fT4 measurement in current practice. This may affect interpretation of results if the follow up samples are collected at different times of the day from the same individual. We aimed to assess the diurnal variation in serum TSH and fT4 levels in healthy adults. Healthy adult volunteers with Sinhalese ethnicity aged 21 -50 years were selected for the study. Subjects with thyroid or any other disease, on medications including oral contraceptives and postmenopausal women were excluded. Two blood samples were drawn from each subject at 8-9 am and 3-4 pm on the same day. TSH and fT4 were measured using VITROS ECi Immunodiagnostic system. Paired T test was used to assess whether there was a statistically significant diurnal variation in hormone values. Data were analyzed using IBM SPSS Statistics Version 20 and Microsoft Excel 2013. A total number of 36 subjects with an equal number of males & females participated in the study. There was no variation in hormone levels due to age or gender. Log transformed TSH and fT4, showed a statistically significant difference for both TSH (p=0.013) and freeT4 (p<0.001) values in the morning and afternoon. Standardization of sample collection time may be important for TSH and fT4 hormone assays as this study revealed a statistically significant diurnal variation for both hormones. However further studies are needed in patients withthyroid diseases and individuals of other age groups as well to ascertain the clinical significance.