Hemangiomas are the most common tumors of infancy, with a prevalence of 5 to 10 percent among one-year-olds. They are characterized by rapid growth in the first year of life, followed by involution and gradual regression by adolescence.¹,² We recently treated a three-month-old infant with massive hepatic hemangiomas and primary hypothyroidism who needed very high doses of thyroid hormone to restore euthyroidism and normal thyrotropin secretion. This finding suggested that the rate of degradation of thyroid hormone was accelerated. We subsequently identified high levels of type 3 iodothyronine deiodinase activity in the hemangioma tissue. This selenoenzyme, normally present in . . .