In an adult non‑pregnant patient with severe overt hyperthyroidism (thyroid‑stimulating hormone < 0.005 µIU/mL, free thyroxine > 100 pmol/L), which foods should be avoided?

Dietary Recommendations for Severe Overt Hyperthyroidism

Patients with severe overt hyperthyroidism should strictly avoid excessive iodine intake from all sources, as iodine exposure can worsen thyrotoxicosis and precipitate thyrotoxic crisis, particularly in those with Graves' disease or autonomous thyroid nodules.

Foods and Substances to Avoid

High-Iodine Foods

• Seaweed and kelp products – These contain extremely high iodine concentrations and represent the most dangerous dietary source 1
• Fish and seafood – Particularly ocean fish, shrimp, and other marine products that concentrate iodine 1
• Iodized salt – Switch to non-iodized salt during the acute hyperthyroid phase 1, 2
• Dairy products – Milk, cheese, and yogurt contain significant iodine from animal feed and disinfectants used in processing 1

Critical Non-Food Iodine Sources

Beyond dietary sources, patients must avoid non-nutritional iodine exposure that can deliver massive iodine loads 3, 2:

• Iodinated contrast agents used in CT scans and other radiologic procedures pose extreme risk and can trigger iodine-induced hyperthyroidism or thyrotoxic crisis 4
• Povidone-iodine disinfectants applied topically to skin 3, 4
• Amiodarone (antiarrhythmic medication) delivers enormous iodine quantities 3, 4
• Iodine-containing supplements and multivitamins 4

Physiologic Rationale

The danger of iodine in hyperthyroidism stems from two mechanisms 4:

1. Volume of autonomous tissue – In Graves' disease and toxic nodules, thyroid tissue functions independently of TSH control
2. Quantity of iodine exposure – Excess iodine provides substrate for unregulated thyroid hormone synthesis

In severe hyperthyroidism with autonomous function, additional iodine substrate allows accelerated production of T3 and T4, worsening thyrotoxicosis 5, 4. This is particularly dangerous because hyperthyroid patients already demonstrate relative T3 overproduction, with T4/T3 ratios of 48:1 compared to 71:1 in euthyroid individuals 5.

Additional Dietary Considerations

While the evidence focuses primarily on iodine restriction, patients should also consider:

• Avoiding excessive caffeine – Can exacerbate tachycardia, tremors, and anxiety already present in thyrotoxicosis 6
• Limiting alcohol – May worsen heat intolerance and can interfere with antithyroid medication metabolism 1
• Avoiding stimulants – Energy drinks and supplements containing stimulants compound sympathetic hyperactivity 6

Clinical Pitfalls

The most critical error is failing to recognize non-dietary iodine sources 3, 4. Patients with severe hyperthyroidism requiring urgent imaging studies face a dilemma: iodinated contrast can precipitate thyrotoxic crisis, yet the clinical indication may be life-threatening. In such cases, prophylactic treatment with antithyroid drugs combined with perchlorate can prevent iodine-induced exacerbation 4.

Do not restrict iodine indefinitely – Once hyperthyroidism is controlled with antithyroid drugs, radioactive iodine, or surgery, normal iodine intake should resume to prevent deficiency 3, 2. The goal is temporary restriction during the acute hyperthyroid phase only.

Monitoring During Dietary Restriction

Assess thyroid function regularly during iodine restriction 3:

• TSH (will remain suppressed initially)
• Free T4 and free T3 (primary targets for monitoring treatment response)
• Consider 24-hour urinary iodine excretion if compliance with restriction is uncertain (target <100 mcg/24hr during restriction phase) 3, 2

Coordinate dietary iodine restriction with pharmacologic therapy – Antithyroid drugs (propylthiouracil or methimazole) remain the cornerstone of treatment, with dietary modification serving as an important adjunct 6, 7. PTU has the additional benefit of blocking peripheral conversion of T4 to T3, which may provide faster symptom relief 5.