Effects of fasting on thyroid hormone profiles: a systematic review and meta-analysis of TSH, FT3, FT4, and total T3...

Fasting induces coordinated metabolic and endocrine adaptations, yet its effects on thyroid hormone dynamics remain incompletely characterized. This systematic review and meta-analysis aimed to evaluate the effects of fasting on thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), free thyroxine (FT4), and total triiodothyronine (total T3).

PubMed, Web of Science, and Scopus were searched for peer-reviewed original research articles published in English between January 2000 and June 2026. Human studies examining fasting interventions lasting ≥ 24 h and reporting pre-post thyroid hormone data in adults were included. Standardized mean changes were calculated as Hedges' g and pooled using random-effects models. Sensitivity analyses were performed using assumed pre-post correlations of r = 0.3, 0.5, and 0.7. To preserve statistical independence, one effect estimate per study (corresponding to the longest eligible fasting duration) was retained in the primary analysis. Subgroup analyses compared fasting durations of ≤ 3 days and > 3 days, and random-effects meta-regression examined fasting duration as a continuous moderator.

Fifteen studies were included in the systematic review. Fasting was associated with significant reductions in FT3 (k = 7; g = -1.979, 95% CI [-3.054, -0.905], p < 0.001), TSH (k = 10; g = -0.581, 95% CI [-0.882, -0.280], p < 0.001), and total T3 (k = 4; g = -1.337, 95% CI [-1.866, -0.807], p < 0.001). No significant change was observed in FT4 (k = 7; g = 0.073, 95% CI [-0.237, 0.382], p = 0.644). Heterogeneity was considerable for FT3, moderate for TSH and FT4, and low to moderate for total T3. Sensitivity analyses produced comparable estimates. Subgroup analysis indicated a larger FT3 reduction after fasting > 3 days, whereas duration subgroup differences were not significant for TSH, FT4, or total T3. Meta-regression showed no significant linear association between fasting duration and any thyroid outcome.

Fasting produces a biomarker-specific thyroid response characterized by pronounced reductions in FT3 and total T3, a moderate decrease in TSH, and relative stability of FT4. This pattern is more consistent with altered peripheral thyroid hormone metabolism and reduced T4-to-T3 conversion than with a uniform suppression of thyroid hormone production.