Serum diiodotyrosine.

Serum diiodotyrosine (DIT) was measured by radioimmunoassay in healthy subjects patients with thyroid disease and a variety of laboratorty animals. Ninety-two healthy adults had a mean level of 101 ng/100 ml. There was no sex difference in DIT levels but DIT fell with aging. There was no change with short term oral SSKI administration. Athyrotic subjects had measurable but reduced levels (mean = 52 ng/100 ml). Hyperthyroid subjects had levels slightly, but not significantly, higher than controls (mean = 149 ng/100 ml). Treatment of hyperthyroidism was followed by a small but significant fall in DIT levels, but there was no change in DIT levels with thyroid hormone therapy of hypothyroidism. A large species variation in serum DIT levels was found among laboratory animals with mean levels ranging from 17 ng/100 ml in mice to 428 ng/100 ml in dogs.