Blood levels of 3,5,3'-triiodothyronine and thyroxine: differences between children, adults, and elderly subjects.

The serum levels of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) in children, adolescents, adults, and elderly subjects have been measured by radioimmunoassays. It was found that while the T4 levels were essentially equal in all age groups examined, the T3 levels were markedly different. In children and adolescents (1-15 years), high values were recorded; indeed, they exceeded the upper normal limit in adults (20-80 years). From the age of 20, the T3 levels remained unaltered until the age of 80, after which there was a further reduction, to values approaching the lower normal limit for T3 in middle-aged subjects. The findings emphasize that separate normal values must be established for different age groups, in order to avoid diagnostic misinterpretations and therapeutic failures.