The cytokine interleukin-1beta (IL-1beta) is an important regulator of thyroid cell function. IL-1 receptors are present on normal thyrocytes, but the signaling pathway is not fully clarified. As the adenylate cyclase is presumably not activated, we have in the present study investigated whether the cGMP pathway was involved in the actions of IL-1beta, whether the effects of IL-1beta on cultured human thyrocytes were reversible, and whether the effects were counteracted by IL-1 receptor antagonist (IL-1ra), a naturally occurring, specific blocker of IL-1 receptors on many cells. TSH-stimulated cultured human thyroid cells exposed for 72 h to IL-1beta (0.0002-20 microg/liter = 1-105 IU/liter) exhibited a dose-dependent and reversible inhibition of thyroglobulin and cAMP release and a dose-dependent stimulation of cGMP and IL-6 release. These effects were counteracted by coincubation with 250 or 125 microg/liter, but not with 25 and 2.5 microg/liter, IL-1ra. IL-1ra by itself inhibited the release of cAMP, but did not modulate the release of thyroglobulin, cGMP, or IL-6 from the thyrocytes, and IL-1ra was not produced in the extracellular compartment. The nitric oxide generator, sodium nitroprusside, dose dependently generated a TSH-independent release of nitric oxide and cGMP from the thyrocytes. These results indicate that all of the studied effects of IL-1beta on cultured human thyrocytes were exerted through activation of the IL-1 receptor with a signaling pathway involving activation of cGMP and inhibition of cAMP.