What supplements affect thyroid function tests?

Supplements That Affect Thyroid Function Tests

Several supplements can significantly alter thyroid laboratory values, with iodine being the most clinically important, followed by supplements containing actual thyroid hormone (desiccated thyroid, tiratricol), and to a lesser extent biotin which can cause laboratory interference.

Iodine Supplements

Iodine supplementation is the most common supplement affecting thyroid labs, with effects varying dramatically based on dose and underlying thyroid status.

Effects in Normal Thyroid Function

• High-dose iodine (1,500 mcg daily) causes measurable changes in thyroid function tests: small decreases in serum T4 and T3 with compensatory increases in TSH, though all values typically remain within normal range 1
• Moderate supplementation (500-250 mcg daily) does not affect thyroid function in euthyroid individuals 1
• Standard prenatal supplement doses (150 mcg daily) are generally safe in iodine-sufficient populations 2

Effects in Thyroid Disease

• In patients with autonomous thyroid nodules, even small iodine supplements (100-400 mcg daily) progressively increase serum T4 and T3, potentially precipitating or worsening hyperthyroidism (Jod-Basedow phenomenon) 3
• In iodine-deficient populations, supplementation programs (150-200 mcg/day) have been associated with increased incidence of iodine-induced hyperthyroidism, subclinical hypothyroidism, and thyroid autoimmunity 4
• Chronic excess iodine exposure induces autoimmune thyroiditis and is associated with elevated TSH, lower thyroid hormone levels, and increased thyroid autoimmunity leading to hypothyroidism and goiter 4

Common Sources of Excess Iodine

• Iodinated contrast agents used for radiologic studies 4
• Topical iodine disinfectants (povidone-iodine) 4
• Amiodarone (anti-arrhythmic medication) 4
• Kelp and seaweed supplements (often contain very high iodine content) 5

Supplements Containing Thyroid Hormone

Nutritional supplements containing actual thyroid tissue represent a serious and underrecognized risk for thyroid dysfunction.

Desiccated Thyroid in Supplements

• Some nutritional supplements sold in health food stores contain desiccated porcine or bovine thyroid 6
• These products can cause frank hyperthyroidism with tachycardia, amenorrhea, and weight loss when consumed at recommended doses, which may exceed normal thyroid replacement doses by 2-3 times 6
• These supplements are largely unregulated by the FDA 6

Tiratricol (TRIAC)

• Tiratricol, marketed as a "metabolic accelerator" and fat loss supplement, causes profound thyroid dysfunction: low TSH with markedly elevated T3 values 7
• Doses of 3,000-6,000 mcg daily cause lethargy, loss of appetite, and muscle weakness 7
• After discontinuation, thyroid levels may take 40 days to 5 months to return to baseline 7

Other Supplements with Limited or No Effect

Selenium

• Selenium supplementation has shown benefit only in specific populations (particularly those with selenium deficiency and autoimmune thyroid disease) with limited generalizability 5
• Routine supplementation does not affect thyroid labs in selenium-replete individuals 5

Zinc

• Similar to selenium, zinc supplementation benefits are limited to specific deficient populations 5
• No significant impact on thyroid function tests in zinc-replete individuals 5

Supplements with No Proven Effect

• Vitamin B12 has little to no evidence of impact on thyroid disorders 5
• Low-dose naltrexone shows no proven effect on thyroid function 5
• Ashwagandha root extract lacks evidence for thyroid impact 5

Dietary Factors

Minimal Impact on Thyroid Labs

• Cruciferous vegetables and soy have little proven effect on thyroid function tests in patients with thyroid disorders 5
• Gluten and dairy avoidance only impacts thyroid labs indirectly by improving levothyroxine absorption in patients with concomitant sensitivities to these substances 5

Clinical Pitfalls and Monitoring

Key caveat: Always obtain a detailed supplement history, as patients often do not consider over-the-counter products as "medications" and may not volunteer this information.

• Check for iodine-containing supplements, particularly in patients with autonomous nodules or pre-existing thyroid disease 3
• Be aware that "natural" or "metabolic" supplements may contain undisclosed thyroid hormone 6, 7
• In populations with high prevalence of thyroid disorders, iodine status should be assessed by 24-hour urinary excretion combined with thyroid function tests 4
• Monitor thyroid function if patients are taking high-dose iodine supplements (>500 mcg daily), especially if they have underlying thyroid abnormalities 1, 3