An interesting paper. Whether thyroid tests need to be done fasting - or not - has been discussed many times.
If we accept this paper, we can see that fasting versus high blood glucose seems to have a potentially very significant effect - with TSH falling and FT4 rising.
Ironically given that we are often told that FT3 varies too much to be useful, the impact on FT3 is relatively slight.
However, the authors come up with a totally nonsensical idea in their conclusions:
Different reference ranges can be used for thyroid values based on whether the patient is in a fasting or fed state.
I simply do not see how you can do that as people consume different foods, in different quantities, and at different times before having their blood drawn. You cannot reduce that to two ranges - you would need many. Effectively, I think you would have to measure blood glucose and use that to dictate which reference range to use or derive a "correction" factor. If you think we have problems now with different labs having different ranges, just imagine that each one had different ranges (or approaches to correction factors) for some or all the various blood glucose levels they try to consider.
Therefore, in my view, there is just one sensible approach. Always have your blood draw fasting and a significant time after your last intake of calories.
My opinion has long been that we should fast for thyroid tests. My rational is that some tests definitely have to be done fasting. INR, blood glucose and many others.
If we always fast then, should we need one of these tests which must be done fasting, then any impact of fasting versus not fasting, even if it is very small, is irrelevant. Because our entire history of blood tests will have been done fasting.
Effect of Pre- and Postprandial Plasma Glucose Levels on Thyroid Hormones: A Cross-Sectional Study
Abstract
Background
There is not much data regarding the effect of plasma glucose on thyroid hormones. Currently, there is no consensus regarding the timing of blood sample collection for thyroid hormones. Evaluation of the correlation between plasma glucose and thyroid hormones may enhance pathophysiological understanding of postprandial thyroid stimulating hormone (TSH) decline.
Objectives
To study the effect of pre-and postprandial plasma glucose levels on thyroid hormones.
Methodology
A cross-sectional study was done among participants aged 18 years and above after obtaining informed consent. Venous blood samples for fasting and postprandial plasma glucose, fasting, and postprandial thyroid profile ((FT3), (FT4), and TSH) were collected. The association was studied using the t-test and chi-square test between groups, while correlation using Pearson’s correlation coefficient. A p-value of <0.05 was considered statistically significant.
Results
Among the 197 participants, 126 (64%) were female and 71 (36%) were male. The mean (± S.D.) of age was 43.74 (± 12.62) years. Mean (± S.D.) postprandial TSH (4.31 μU/ml (± 7.79)) and free T3 (285.52 pg/dl (± 32.46)) were less than the fasting values (4.99 μU/ml (± 7.78)) and (295.84 pg/dl (± 32.56)). The mean (± S.D.) of both fasting and postprandial TSH and free T4 were less in the high plasma glucose group compared to the normal plasma glucose group (fasting state p-values 0.005, <0.0001 & postprandial state p-values 0.016, <0.0001). The correlation between fasting plasma glucose and fasting TSH values (Pearson correlation; r=-0.18; p-value 0.232) was observed across all the plasma glucose ranges.
Conclusion
There is a significant postprandial decline in TSH levels than the fasting TSH, indicating that there is a plasma glucose-mediated reduction in TSH values.
Full paper open access here:
Thyroid T4-T3 conversion is strongly linked to insulin resistance
