- Design
- Systematic review and meta-analysis of paired same-day samples
- Population
- 1,257 participants across 14 studies
- Primary outcome
- Difference in TSH, free T4 and free T3, postprandial minus fasting
- Effect
- TSH −0.80 mIU/L (95% CI −1.07 to −0.53); no difference in free T4 or T3
A meta-analysis pooled 14 studies in which the same people had thyroid tests fasting and after eating on the same day, 1,257 participants in all.
TSH was on average 0.80 mIU/L lower after a meal (95% CI 0.53 to 1.07), with larger falls in overt (about 2.5 mIU/L) and subclinical hypothyroidism (about 2.2 mIU/L). Free T4 and free T3 did not differ. Heterogeneity was extreme (I² 99.7%) and certainty low.
The practical change is small and cheap. For most people the difference will not matter, but for a borderline TSH a non-fasting sample could push a subclinical hypothyroid result into the reference range. Recording fasting status, and repeating borderline results under the same conditions, makes serial values comparable.
- Add fasting status to thyroid request forms or sample labels
- Repeat a borderline TSH under the same fasting conditions before reclassifying
- Consider a fasting morning sample when screening for subclinical hypothyroidism
- Flag on reports when fasting status was not recorded for borderline results
Why it matters
Unrecorded fasting status may quietly reclassify borderline hypothyroidism as normal.
Don't overread it
Low-certainty evidence; it does not show that fasting samples are more accurate, only that the two differ.
The statistics, in plain English
A 0.8 mIU/L fall is small for most people but can move a value of, say, 5 mIU/L into the normal range. An I² of 99.7% means the studies varied enormously, so the exact size is uncertain.
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