Serum cyclic 3',5'-nucleotide phosphodiesterase in patients with various thyroid disorders.

Previous observations that cyclic 3',5'-nucleotide phosphodiesterase activity exists in mammalian sera including human serum prompted us to investigate the phosphodiesterase levels in sera of patients with various thyroid disorders. Both serum cyclic AMP phosphodiesterase (cAMP-PDE) and cyclic GMP phosphodiesterase (cGMP-PDE) activities measured in a low substrate concentration were elevated 3-fold in subacute thyroiditis and slightly in hyperthyroidism, compared to the normal. Slight decreases of these enzyme activities were observed in primary hypothyroidism. PDE activities were positively correlated with the value of T3-RSU and serum thyroid hormone levels in hyper- and hypothyroidism. Altered enzyme activities returned to normal during the course of recovery. Identical results were obtained when plasma was tested. These results suggest that serum PDE activities may be partly related to the thyroid function.