Does Time of Sampling or Food Intake Alter Thyroid Function Test?

CONTEXT A common question from most patients or laboratories is whether blood sample for thyroid-stimulating hormone (TSH) and free T4 (fT4) needs to be collected in a fasting state and whether time of the day when sample is collected matters. AIMS The aim of the study was to study the impact of the time of day and food intake on levels of TSH and fT4. SETTINGS AND DESIGN Cross-sectional prospective data collection. SUBJECTS AND METHODS We prospectively collected data from 52 volunteers who were not known to have any thyroid disorder and were not on any thyroid-related medication. Blood samples for TSH and fT4 were collected on day 1 at 8 am and 10 am with the patient remaining in the fasting state till the collection of the second sample at 10 am. On day 2, samples were collected at 8 am (fasting state) and at 10 am (2 h postprandial state). In 22 volunteers from the group, the tests were performed in three common assay techniques including chemiluminescent assays (chemiluminescent immunoassay [CLIA] and chemiluminescent microparticle immunoassay [CMIA]) and enzyme-linked fluorescence assay. RESULTS The mean (standard deviation) and median (interquartile range) TSH during the extended fast on day 1 were 2.26 ± 1.23 and 2.19 (1.21-3.18), which was significantly lower than the fasting TSH performed on day 1 (P < 0.001). Similarly, the values of TSH 2 h postmeal on day 2 of the testing (mean 1.93 ± 1.12; median 1.64 [1.06-2.86]) were significantly lower than TSH performed in the fasting state on day 2 (P < 0.001). The mean fT4 value was 1.01 ± 0.15 with median of 0.99 (0.91-1.11) in the fasting state and there was no significant difference between the fT4 values performed during fasting, extended fasting, and postmeal state. Among the volunteers in whom the test was performed in the three different assay techniques, the TSH was not statistically different either in the fasting (P = 0.801), extended fasting (P = 0.955), and postprandial samples (P = 0.989). The fT4 values did not vary significantly when done by the same assay method. However, the fT4 levels varied significantly (P < 0.001) when done by another assay method. CONCLUSIONS We conclude stating that the timing of the test affects TSH values and this should be factored in making decisions in diagnosis of subclinical hypothyroidism.