Niacin Does Not Cause Thyroid Problems at Standard Doses
Niacin (vitamin B3) does not cause thyroid dysfunction at physiological or standard supplemental doses, and the available evidence does not support a causal relationship between niacin supplementation and thyroid disease. The guideline literature extensively documents niacin's adverse effects—including flushing, hepatotoxicity, and gastrointestinal disturbances—but thyroid dysfunction is notably absent from these comprehensive safety profiles 1, 2.
Evidence from Guidelines
The most recent and authoritative guidelines provide no indication that niacin causes thyroid problems:
The 2022 ESPEN Micronutrient Guideline comprehensively reviews niacin safety, establishing an upper limit of approximately 900 mg/day for adults based on flushing as the primary adverse effect, with no mention of thyroid dysfunction 1.
Well-documented adverse effects at therapeutic doses include cutaneous flushing (most common), hepatotoxicity (particularly at ~3 g/day), gastrointestinal disturbances, and elevated liver enzymes 1, 2, 3.
The 2019 American Diabetes Association guidelines discuss niacin's cardiovascular effects and note increased risk of new-onset diabetes and gastrointestinal/musculoskeletal side effects, but do not identify thyroid dysfunction as a concern 1.
Clarifying the Iodine-Niacin Confusion
The limited research suggesting a thyroid connection involves iodine-containing niacin preparations (Iodo-Niacin), not niacin itself:
One 1984 case report documented iodine-induced hyperthyroidism from "Iodo-Niacin," where the iodine component—not the niacin—caused the thyroid dysfunction 4.
This is consistent with well-established mechanisms of iodine-induced thyroid disease in patients with underlying thyroid abnormalities or autonomous nodules 4, 5.
Niacin's Actual Thyroid Interactions
Research shows niacin may have beneficial effects on thyroid treatment, not harmful ones:
Studies in animal models suggest niacin increases thyroid radiosensitivity to radioiodine therapy and may decrease serum thyroid hormone levels (primarily total T4 and TBG), potentially serving as a radiosensitizer 6, 3.
These effects are being investigated for therapeutic applications in radioiodine treatment of hyperthyroidism and large goiters, not as adverse effects 6.
Clinical Bottom Line
Monitor for established niacin adverse effects—flushing, hepatotoxicity, and gastrointestinal symptoms—but thyroid dysfunction is not a recognized complication of niacin supplementation 1, 2. If thyroid problems develop in a patient taking niacin, investigate other causes rather than attributing it to the niacin itself. The only exception would be if the patient is taking an iodine-containing niacin preparation, in which case the iodine component requires evaluation 4.