This report describes a systemic immediate-type allergic reaction to commercial thyrotropin in a patient with hypothyroidism and chronic candidiasis. Immunologic studies of the patient's serum revealed precipitating antibodies against bovine albumin and gamma globulin, and the thyrotropin preparation was found to be contaminated with these proteins. Skin testing with purified bovine albumin and commercial thyrotropin produced wheal-and-flare reactions indicating the presence of reaginic antibodies but did not evoke Arthus or delayed-type responses. The patient's serum contained elevated concentrations of thyrotropin consistent with the diagnosis of primary hypothyroidism, but it did not inhibit the biologic activities of human or bovine thyrotropins. Although antibodies against bovine proteins are often found in human serum, they are only rarely associated with disease. In our patient a systemic allergic reaction was apparently the result of the interaction between anti-albumin antibodies and the albumin contaminating the TSH. The studies of the effects of the patient's serum on the biologic activities of bovine or human thyrotropins did not provide evidence for an anti-TSH that could be implicated in the pathogenesis of hypothyroidism. © 1973.