SummaryThe mechanism for the alteration in cartilage growth and metabolism present both in childhood thyroid hormone deficiency states and in hyperthyroid children has not been defined. Since thyroid hormone has a direct effect on the adenylate cyclase-cyclic 3′,5′-nucleotide phos-phodiesterase system in certain tissues, the effect of thyroid hormones on soluble cyclic 3′,5′-nucleotide phosphodiesterase (PDE) activity prepared from chicken epiphyseal (growth) cartilage was examined. L-Triiodothyronine at a concentration of 1 × 10−4M resulted in 69% inhibition of soluble cartilage cyclic 3′,5′-nuc-leotide phosphodiesterase activity. At identical concentrations L-thyroxine, D-thyroxine, D-triiodothyronine, and L-diiodothyronine demonstrated only 42, 41, 38, and 20% inhibition of phosphodiesterase activity, respectively. The concentration of L-triiodothyronine that resulted in 50% inhibition of phosphodiesterase activity was 3.5 × 10−5M. This investigation demonstrates a significant inhibitory effect of L-tr...