Propylthiouracil, ipodate, dexamethasone and periods of fasting induce different variations in serum rT3 in dogs.

Fasting or administration of propylthiouracil (PTU), ipodate, or dexamethasone are all known to induce a pattern of low serum triiodothyronine (T3) concentrations and high serum reverse T3 (rT3) concentrations in humans. In the present study it was found that this is not a universal phenomenon. In normal dogs exposed to fasting or these various pharmacologic agents, the serum T3 level was always depressed as in humans. However, different variations in serum rT3 levels were observed. Fasting and PTU administration were accompanied by slight decreases in the serum rT3 concentration. A single dose of ipodate did not alter serum rT3 levels, but serum thyroxine (T4) levels increased by more than 50%. Dexamethasone induced a considerable increase in serum rT3 levels, while serum T4 levels were unaltered. The results suggest that the high serum rT3 level nearly always seen in "the low T3 syndrome" in humans is merely a coincidental character of the human species, and that it has little importance for in vivo homeostasis.