Supplies of potassium iodide, a preventive against radiation poisoning of the thyroid gland, are running low at some manufacturers, as Americans seek protection amid fears that radiation from Japan could head to the U.S., according to the companies.

One leading supplier, Anbex Inc., quickly sold out of its supply of more than 10,000 14-tablet packages on Saturday, said Alan Morris, president of the Williamsburg, Va., company.

He...