Hyperthyroidism increases metabolic rate, mitochondrial ATP production and protein synthesis but it remains to be determined whether all tissues and and synthesis of specific protein pools are equally affected by hyperthyroidism. Previous studies showed that mitochondrial function was less responsive to elevated T3 levels in the low-oxidative plantaris muscle compared to other tissues in rats. We tested the hypothesis that in T3-treated animals mitochondrial protein synthesis would increase in oxidative but not glycolytic tissues. Male rats received either T3 (200 µg/d, n=10) or saline (controls, n=9) by subcutaneous pump for 14 d and then in vivo protein synthesis rates were measured using [¹⁵N] phenylalanine in liver, heart, plantaris and red gastrocnemius (Red Gast). Mitochondrial protein synthesis rate in T3-treated rats was higher than controls by 62% in Red Gast and plantaris and 89 and 115% in liver and heart, respectively (p<0.01). Cytoplasmic protein synthesis rates in the T3 group were 107-176% higher than control values (p<0.01). There was also indirect evidence that protein breakdown was increased in all tissues of the T3-treated rats. Phosphorylation of selected regulators of protein synthesis in plantaris and Red Gast (mTOR, p70S6 kinase, 4E-BP1), however, were not significantly affected by T3. We conclude that T3-infusion stimulates a general increase in mitochondrial and cytoplasmic protein synthesis rate among tissues and that this does not appear to explain the tissue-specific responses in mitochondrial oxidative capacity.