Effect of 3, 5, 3-triiodothyronine and 3, 3, 5-triiodothyronine administration on serum tri-, di-, and monoiodothyronines and plasma cyclic nucleotides in sheep

T₃ and/or rT₃ were administered to sheep in order 1) to determine their influence on plasma cyclic nucleotides and 2) to examine their effects on serum T₄, T₃, rT₃, 3, 5-diiodothyronine (3, 5T₂), 3, 3T₂, and 3-monoiodothyronine (3T₁). The animals were divided into five groups, and each group received one of the following treatments: 1) placebo, 2) low T₃ (1.5 µg/kg), 3) high T₃ (4.5 µg/kg), 4) high rT₃ (4 µg/kg), or 5) low rT₃ (2.5 µ/kg) plus low T₃ (1.5 µ/kg) in combination. Treatments were administered every 8 h for 6 days, and the animals were studied throughout this period. Cyclic nucleotides and iodothyronines were measured by RIA. High dose T₃ treatment increased plasma cAMP by 52% (P < 0.02), whereas plasma cGMP was unaffected. Other treatments did not alter plasma cyclic nucleotides. Both low and high dose T₃ treatment decreased T₄ and rT₃ levels, whereas rT₃ treatment did not alter levels of serum T₄ or T₃. Serum 3, 3T₂ concentrations were increased by rT₃ and T₃ treatment, both singly and in combination. Serum 35T₂ levels were undetectable (<0.055 nM), except after rT₃ administration. 3T₁ levels were unchanged by treatments, except that high T₃ administration slightly, but significantly, decreased serum 3T₁ concentrations. These results indicate that in the sheep 1) T₃ markedly increases plasma cAMP and decreases T₄ and rT₃ levels, whereas rT₃ administration is inactive in both of these systems; 2) neither T₃ nor rT₃ alters plasma cGMP; 3) degradation of rT₃ is directed relatively more to 3, 3T₂ than to 35T₂; 4) metabolism of both T₃ and rT₃ contribute comparably to 3, 3T₂ levels; and 5) T₃ may enhance the conversion of rT₃ to both 3, 3T₂ and 3, 5T₂.