Graft-versus-host reactions within the thyroid gland may evoke pari passu autoimmune thyroiditis.

Unilateral, intraorganal injection of parental lymph node cells into thyroid lobes of F1 hybrid rats elicited an intense, local graft-versus-host reaction within the injected lobes containing massive numbers of lymphocytes. Surprisingly, significant mononuclear cell infiltration was also seen within the contralateral (noninjected) thyroid lobes, even when the isthmus had been divided. The local graft-versus-host reaction spread from the injected gland to involve the regional lymph nodes and spleen. Migration of 51-cr-labeled donor lymph node cells was documented in these same tissue sites; lesser numbers of labeled cells were found in distant lymph nodes, including the contralateral cervical nodes. However, no significant label was found in the contralateral thyroid lobes. Microscopic evaluation of noninjected glands revealed intense infiltration of mononuclear cells around and within follicles, reminiscent of lesions induced by immunization with thyroid extract in Freund's complete adjuvant. The absence of donor cells in the inflammatory reactions within noninjected thyroid lobes raises the possibility of an autoimmune rather than graft-versus-host pathogenesis.