Serum TSH variability in normal individuals: the influence of time of sample collection

Difficulty in treatment decisions can arise when TSH levels measured on the same patient on the same day but at different times show considerable variability. This study was a prospective, observational evaluation of 100 consecutive adult patients who had serum TSH tests ordered by attending physicians at an outpatient clinic. Early morning fasting serum TSH levels were compared to late morning non-fasting serum TSH levels in the same patients on the same day The late morning non-fasting TSH tests declined in 97 of 100 subjects by an average of 26.39% when compared to early morning, fasting, TSH test results. This lead to reclassification of 6% of patients from presumptive subclinical hypothyroidism to "normal." Since the time of day of phlebotomy or the fasting or non-fasting status of the patient, or both, can significantly affect serum TSH test results, the diagnosis of subclinical hypothyroidism should not be made only on a fasting TSH measurement. Further studies are needed to determine the independent effects of the time of phlebotomy and fasting/non-fasting status on TSH levels.