Persistent Low Concentration of Human Chorionic Gonadotropin in a Nonpregnant Woman

Abstract

A 48-year-old woman presented for radio-iodine ablation therapy 3 months after undergoing a complete thyroidectomy performed for compressive goiter symptoms. The patient’s medical history included stage-3 follicular variant papillary thyroid cancer with no nodal involvement and no metastatic disease. A 6.5-cm papillary carcinoma had been identified and removed surgically. Despite this surgical treatment, 3 foci were detected by 125I uptake testing, a finding that prompted the use of therapeutic ablation. Because the patient reported 5 months of amenorrhea, a quantitative serum human chorionic gonadotropin (hCG) test (hCG+β assay, Roche Diagnostics) was performed to rule out pregnancy. The result was 7.0 IU/L (reference interval: <5.0) and the administration of 100 mCi I131 was cancelled owing to concern for a potential pregnancy. A repeat hCG test performed 4 days later was 7.0 IU/L. Follicle stimulating hormone (FSH) was determined to be 110 IU/L [reference intervals: 1.9–11.6 (follicular phase), 1.4–9.6 (luteal phase), and 21.5–131 IU/L (postmenopausal)]. hCG is produced by the trophoblastic tissue of the placenta and hence is a reliable marker for pregnancy. hCG also has clinical utility as a tumor marker because of its production by trophoblastic and nontrophoblastic neoplasms. Finally, hCG is also produced by the gonadotropic cells of the pituitary during the menopause (1)(2). As demonstrated by this patient, a persistent low concentration of hCG is particularly perplexing. Such cases have 3 potential etiologies: (a) false-positive hCG due to interfering antibodies, (b) quiescent gestational trophoblastic disease, and (c) pituitary hCG. ### false-positive hCG due to interfering antibodies There are several reports of falsely increased hCG due to endogenously produced interfering antibodies (3). Interfering antibodies can be of 2 types: human antianimal antibodies (HAAA) or heterophile antibodies. HAAAs are directed toward a specific antigen and may be produced after treatment with therapeutic antibodies or exposure to animal antigens (4). Heterophile antibodies display …