A static x-ray fluorescent technique utilizing an 241Am source of low activity and a wide view diverging collimator which allows in vivo determination of the concentration and total content of iodine in the thyroid is described. Determinations of normal thyroidal iodine concentration and total iodine content were as follows: 0.40 mg/g and 10.6 mg for 16 men, and 0.67 mg/g and 17.6 mg for 14 women. This fluorescent system has been applied to 70 patients to date. Thyroidal iodine concentrations in patients with Graves' disease were approximately equal to those in healthy suspects, and little change was observed among the concentrations in patients before, during, or after treatment. In most of the patients with chronic or subacute thyroiditis, concentrations levels were lower.