Melatonin Does Not Cause Thyroid Dysfunction in Humans
Based on available evidence, melatonin supplementation does not cause clinically significant thyroid dysfunction in humans and can be used safely without concern for thyroid-related adverse effects. The FDA-approved melatonin labeling does not list thyroid dysfunction as a known side effect 1.
Evidence Analysis
Human Data
The provided evidence contains no human studies demonstrating that melatonin supplementation causes thyroid problems. The USPSTF thyroid screening guidelines 2 discuss risk factors for thyroid dysfunction extensively—including medications like amiodarone—but melatonin is notably absent from this list of causative agents.
A 2022 human study specifically examined whether daytime melatonin ingestion (6 mg) affected thyroid hormone responses in healthy active males 3. The results showed:
- No effect on free thyroxine (fT4) levels
- No effect on TSH responses to exercise
- Melatonin did not alter thyroid function even when combined with physiological stress (submaximal exercise)
Animal Studies Show Complex, Non-Translatable Effects
The animal research reveals dose-dependent and duration-dependent effects that do not translate to human clinical practice:
- A 2024 meta-analysis of rat studies 4 found effects varied by dose and duration, with decreased thyroid hormones at high doses for 4-5 weeks, but increased levels with longer therapy
- Studies showing thyroid suppression used doses and durations far exceeding typical human supplementation 5, 6
- These animal models involved species with seasonal reproductive patterns (turtles 7) or used melatonin to study hypothyroidism treatment 8, 9, not as a cause of dysfunction
Clinical Bottom Line
You can reassure patients that melatonin supplementation at typical doses (1-10 mg) does not cause thyroid problems. The animal data showing thyroid effects:
- Used supraphysiologic doses
- Involved prolonged administration periods
- Studied species with different melatonin-thyroid interactions than humans
- Have not been replicated in human studies
Important Caveats
- If a patient develops thyroid symptoms while taking melatonin, investigate other causes first—the most common causes of hypothyroidism remain Hashimoto's thyroiditis, and hyperthyroidism is typically due to Graves disease or nodular disease 2
- Standard thyroid dysfunction risk factors (female sex, advancing age, family history, autoimmune conditions) remain the primary considerations 2
- No routine thyroid monitoring is needed for patients taking melatonin supplements