Extrathyroidal physiology of monoiodotyrosine in humans.

Normal serum monoiodotyrosine (MIT) levels (n = 152) were 0.69 +/- 0.20 nmol/l. There was wide variation of MIT levels in a 24-hour period without diurnal pattern, and there was no change throughout the menstrual cycle. MIT levels declined upon aging, but levels in hypo- and hyperthyroidism were not significantly different. MIT levels were detected in athyrotic patients (0.32 +/- 0.08 nmol/l). Desiccated thyroid raised the athyrotic MIT levels to the normal range, while levothyroxine did not. Diiodotyrosine (DIT) infusion caused an MIT rise which paralleled but lagged 1 h behind the DIT rise. These data suggest thyroidal as well as nonthyroidal sources of MIT, one of which is deiodination of DIT. Ingestion of 1 g MIT increased serum MIT to 10.6 +/- 1.7 mumol/l in women, and 7.1 +/- 2.3 mumol/l in men 30 min after ingestion; the serum half-life was 45 min.