Higher Prevalence of "Low T3 Syndrome" in Patients With Chronic Fatigue Syndrome: A Case-Control Study.
Ruiz-Núñez, Begoña, Tarasse, Rabab, Vogelaar, Emar F et al. · Frontiers in endocrinology · 2018 · DOI
Quick Summary
This study compared thyroid hormone levels in 98 ME/CFS patients with 99 healthy controls and found that ME/CFS patients were more than twice as likely to have low levels of an active thyroid hormone called T3. The patients also showed signs of chronic low-grade inflammation and had more reverse T3 (an inactive form), which may explain why their bodies aren't producing or using thyroid hormones efficiently.
Why It Matters
If confirmed, these findings could explain the pervasive fatigue and metabolic dysfunction experienced by many ME/CFS patients and might identify a treatable subgroup. This opens the possibility of targeted thyroid hormone or iodine supplementation trials, which could represent a novel therapeutic approach for a condition with few established treatments.
Observed Findings
ME/CFS patients had significantly lower free T3 (difference of 0.1%), lower total T4 (11.9% lower), and lower total T3 (12.5% lower) compared to controls despite similar TSH levels.
Reverse T3 was elevated (13.3% higher) in ME/CFS patients, suggesting a shift from active T3 to inactive reverse T3.
Low circulating T3 below the reference range was found in 16/98 ME/CFS patients versus 7/99 controls (2.56 times higher odds).
Evidence of chronic low-grade metabolic inflammation was present, with thyroid parameters correlating positively with C-reactive protein in ME/CFS patients.
Iodine levels were significantly lower in ME/CFS patients (27.6% lower 24-hour urinary iodine).
Inferred Conclusions
A subgroup of ME/CFS patients exhibits a pattern resembling non-thyroidal illness syndrome and low T3 syndrome, with reduced tissue T3 availability despite normal TSH.
The pattern of low T3 and elevated reverse T3 may reflect a hypometabolic state linked to chronic low-grade metabolic inflammation.
Thyroid supplementation trials (T3 and/or iodide) may be warranted if these findings are confirmed in larger studies.
Remaining Questions
Does the low T3 pattern reflect a primary thyroid dysfunction, a metabolic adaptation to chronic inflammation, or both—and what is the mechanistic relationship?
What This Study Does Not Prove
This study does not prove that low T3 syndrome causes ME/CFS or that thyroid hormone replacement would improve symptoms—it demonstrates an association only. The cross-sectional design cannot establish causation or determine whether the low T3 is a primary problem or a secondary consequence of chronic inflammation. The findings require replication and extension before clinical recommendations can be made.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →