SAT-402 - Wolff-Chaikoff Effect from Iodine Supplements: A Diagnostic Pitfall in Thyroid Uptake Scans

Intro
In the event of iodine surging above physiological intracellular levels, thyroid gland has a protective mechanism, called Wolff-Chaikoff, whereby the gland shuts down hormone synthesis by stopping TPO activity, inhibiting organification. This can be seen with iodine containing medications (amiodarone), foods (seaweed, seafood) and over the counter iodine containing supplements (Lugol’s solution). It is important to recognize this classic but under-recognized pitfall that iodine levels in the body can fluctuate with recent dietary intake or recent iodine containing contrast; therefore RAIU scans in these settings may give discordant results, delaying diagnosis.

Case
The patient is a 78-year-old female with history of subclinical hyperthyroidism, Grave disease now in remission, thyroid nodules, atrial fibrillation and severe osteoporosis who presented in October 2025 for management of thyroid disease, with symptoms of fatigue and weight loss.

She was diagnosed with Graves in her 30s and could not tolerate antithyroid medications due to adverse effects. While definitive treatments were offered to her, she declined and preferred periodic monitoring. Notably, she developed atrial fibrillation more than 10 years ago but managed without the use of amiodarone.

Labs in 2025 showed normal TSI, TSH receptor antibody (TRab), TPO antibodies. Despite this, TSH has been persistently suppressed with free T4 ranging from 1.1 to 1.9 ng/dL (0.60 - 1.89 ng/dL). Prior uptake and scan in February 2023 showed normal range uptake. However, a repeat scan in October 2025, done to evaluate need for treatment, showed decreased uptake at 2.8% and 4.7% at 2 hour and 24 post iodine intake, inconsistent with her labs showing mid to high normal free T4. On further questioning, she endorsed taking over the counter iodine supplements and a higher amount of kelp in her diet than usual.
Iodine levels collected October 7th, 2025 were elevated at 475.2 ug/L (40.0 - 92.0 ug/L). She was counseled on avoiding iodine supplements and reducing kelp in her diet.

Conclusion
This case illustrates how excess iodine intake can lead to misleadingly low RAIU results despite biochemical hyperthyroidism, potentially delaying diagnosis and appropriate management. Wolf-Chaikoff is a well described phenomenon by which thyroid hormone synthesis is paused and iodine uptake is reduced to prevent a hyperthyroid state. Awareness of this phenomenon is critical in preventing misinterpretation of results. Patients should be counseled on avoiding excessive iodine supplements of iodine rich foods before obtaining these scans. Clinicians should keep this in the differential when RAIU results are discordant with other labs and clinical picture. Lastly, this case highlights the importance of a thorough dietary and supplement history prior to thyroid imaging. Repeat imaging with low iodine diet should be pursued to determine a clear etiology.

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