Neurobiology and pharmacology of Huntington's disease.

Reverse triiodothyronine (rT₃) which is apparently a product of 5-monodeiodination of thyroxine (T₄) at the periphery, was measured in 199 chickens of various strains. rT₃ was virtually absent in young birds (less than 1 week to 8 weeks of age) in marked contrast to the elevated rT₃ levels found in human and other mammalian neonates. At one to two years of age there was a significant increase in the number of birds with detectable rT₃. However, rT₃ concentrations were low and often close to the detection limits of the assay in contrast to significant rT₃ levels found in mammals (man, monkey, sheep, rat and dog). An apparent sex difference in relation to rT₃ formation was noted; 46.4% of 97 females and 9.3% of 54 males showed detectable rT₃ levels. The observations described suggest a species difference in regard to peripheral T₄ monodeiodination between birds and mammals.