Iodine deficiency vs. iodine excess: Which is worse?

The recent AACE Annual Meeting and Clinical Congress included many excellent sessions, with only a few that did not live up to my expectations. One of the best plenary lectures was delivered by Michael Zimmermann, MD, and was titled, “Iodine Deficiency and Excess: Worldwide Status.”

I listened most attentively to the reports on iodine deficiency but was in for quite a surprise when I returned to clinic a few days later.

My patient is a middle-aged man who entered the exam room in a wheelchair. He was sweating profusely and shaking all over. He and his wife first noticed that something was wrong a few months earlier. He has been under the care of excellent neurologists for 2 to 3 months without a clear-cut diagnosis.

It was clear by history and physical that he had thyrotoxicosis — almost “thyroid storm” — and was immediately admitted to the hospital. He was taking methimazole 5 mg daily for hyperthyroidism. Propranolol and steroids were added to his therapy and methimazole continued at the 5-mg dose. We convinced ourselves that he was gradually improving and, indeed, his lab values began to decrease.

At admission, thyroid-stimulating hormone (TSH) was 0.01 mcIU/mL (reference range, 0.10 mcIU/mL to 5.00 mcIU/mL) and remained so. Free T₄ had been measured 12 days earlier and was 2.41 ng/dL (reference range, 0.71 ng/dL to 1.85 ng/dL). On that same date, free T₃ was 8.1 pg/mL (reference range, pg/mL (2.2 pg/mL to 4.0 pg/mL).

On admission, free T₄ was 1.6 ng/dL and was 1.66 ng/dL on day 3. Free T₃ was 6.7 pg/mL at admission and went down to 4.6 pg/mL on day 3.

Thyroid antibodies were also elevated: anti-thyroperoxidase was 240 IU/mL (reference range, 0 IU/mL to 35 IU/mL); TSH receptor antibody was 27.09 IU/L (reference range, <1.75 IU/L); and thyroglobulin was 108.0 ng/dL (reference range, 0.3 ng/dL to 49.7 ng/dL).

The clinical diagnosis of iodine-induced hyperthyroidism is well documented, but I cannot yet convince myself that his tremors — more noticeable on the right arm and leg — can be solely attributed to this condition. He is getting better and I will be pleasantly surprised if he restores himself to his pre-tremor state.

Thanks to a careful history taken by the assigned resident, we learned that the patient had been appropriately concerned by his mood and symptoms and, on the advice of family and friends, has been taking large doses of iodine-containing tablets that had been broadcast on the Web as essential therapy when things aren’t going well.

The take home message from Zimmermann’s presentation is that too little iodine intake is not a good idea. In contrast, too much of a good thing is also no good for anyone.