Paraneoplastic eosinophilia. Report of a case with in vitro studies of hemopoiesis.

Eosinophilia is an unusual paraneoplastic manifestation. The authors describe for the first time the association of eosinophilia and metastatic medullary carcinoma of the thyroid. The eosinophil count normalized during remission after chemotherapy but increased at time of relapse. The patient's serum enhanced eosinophilic differentiation of human multipotential hemopoietic precursors but was inhibitory to in vitro growth of murine myelopoietic precursors. The number of multipotential colonies which could be cultured in vitro was lower in the patient's peripheral blood than in normal controls. The authors conclude that a tumor-related circulating factor caused eosinophilia by inducing eosinophilic differentiation and inhibiting myeloid differentiation. Such a mechanism of paraneoplastic eosinophilia has not been previously described.