How Ramadan Fasting Affects Thyroid Function — What Patients and Clinicians Need to Know

Intermittent and time-restricted fasting protocols have attracted enormous interest in longevity and metabolic health research. Ramadan fasting — practiced by hundreds of millions of adults annually — represents one of the largest real-world experiments in prolonged daily fasting, making it a valuable lens through which to examine how the endocrine system adapts to altered meal timing and circadian cues.

This narrative review systematically searched PubMed/MEDLINE and Google Scholar through July 2026, incorporating original research, cohort studies, randomized trials, systematic reviews, and clinical guidance documents. Evidence was stratified by thyroid status: healthy individuals, hypothyroid patients on levothyroxine, and hyperthyroid patients.

In healthy adults, Ramadan fasting preserved hypothalamic-pituitary-thyroid (HPT) axis function within normal physiological reference ranges, suggesting robust homeostatic resilience to meal-timing shifts and caloric restriction. In hypothyroid patients receiving levothyroxine, multiple studies documented a post-Ramadan rise in TSH, most pronounced in those with higher baseline TSH or suboptimal pre-Ramadan control. Crucially, free T4 remained largely stable, indicating the biochemical shift may reflect timing and adherence disruptions rather than true thyroid dysfunction. No levothyroxine regimen — whether taken at suhoor (pre-dawn meal), iftar (sunset meal), or at another interval — consistently outperformed others; patient preference and adherence emerged as the most clinically meaningful determinants. For hyperthyroid patients, direct clinical evidence is sparse, and guidance is expert-opinion based; fasting may be considered in stable, well-controlled individuals but is inadvisable in active thyrotoxicosis.

For a longevity-focused audience, thyroid function is a meaningful healthspan variable — subclinical hypothyroidism correlates with cardiovascular risk, cognitive decline, and metabolic dysfunction in aging adults. Understanding how fasting protocols interact with thyroid status and medication pharmacokinetics is directly applicable to clinical management of time-restricted eating in middle-aged and older adults. The review calls for controlled trials in hyperthyroidism and studies evaluating alternative levothyroxine regimens during fasting periods.