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

Background 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). Methods 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. Results 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. Conclusions 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.