SUN-239 A Case with Elevated Serum Follicle-Stimulating-Hormone (FSH) Concentrations Due to Macro-FSH

[Case report] A 64-year-old woman visited our hospital with the complaint of general malaise and anxiety lasting for two years. She had been diagnosed with idiopathic hypoparathyroidism because of hypocalcemia, hyperphosphatemia, low levels of intact PTH and calcification in the basal ganglia and was taking vitamin D and calcium phosphate. Endocrine workup disclosed high serum FSH [713mIU/mL (Elecsys), reference range for postmenopausal women: 10.5 - 142.8mIU/mL]. Other FSH assay platforms also showed high serum FSH levels: 523mIU/mL for Centaur and 457mIU/mL for Architect. Serum LH and E2 concentrations were 24.8mIU/mL and <11.8 pg/mL, respectively. Human anti-mouse antibody (HAMA) was not detected in the serum. [Methods] Large molecular weight FSH was precipitated using 12.5% polyethylene glycol (PEG) and the precipitation ratio (serum FSH - free FSH) / serum FSH was determined. The molecular size of FSH and IgG-bound FSH were examined using gel filtration chromatography (GFC) and protein G column (PG), respectively. Binding study was performed by incubating (125)I-FSH (24000 cpm) with the serum and the specificity was examined by adding unlabeled FSH, LH, α-subunit of hCG, β-subunit of hCG and TSH. FSH was measured by enzyme immunoassay (EIA). We also examined these parameters in sera from 10 females with serum FSH>70 mIU/mL as controls. [Results] The dilution curve of the patient’s serum FSH was in parallel with that of reference one. The precipitation ratio by PEG was significantly elevated (95.4% vs. 20.3 ± 8.0% in controls). On GFC, a large proportion of serum FSH (89.4%) was eluted in the position of molecular weight > 150kDa, whereas FSH was mostly eluted at 36kDa in controls. Incubation of reference FSH (300mIU and 600mIU) with the serum (100µL) increased the peak levels of the large molecular weight FSH to be 84.0mIU/mL and 180.0 mIU/mL, respectively, from 21.6mIU/mL without exogenous FSH. Serum FSH bound to PG at a significantly higher ratio (87.3%) than that in controls (1.2 ± 1.4%). A significantly higher amount of (125)I-FSH bound to the serum (5691 cpm) than that in controls (704 ± 170 cpm). The binding was displaced with an excess amount (600IU/mL) of unlabeled FSH to be 821 cpm (14.4%) and Scatchard analysis revealed the presence of anti-FSH autoantibodies (Ka 3.0 x 10(7) M(-1), Bmax 0.92 x 10(-8) M). The binding was slightly displaced by other hormones that shared α-subunit; 87.6% by LH, 90.4% by hCGα, 81.1% by hCGβ and 71.8% by TSH. [Conclusion] We report a woman with high serum FSH concentrations due to macro-FSH caused by anti-FSH autoantibodies. The possibility of macro-FSH should be considered in a case with unexpectedly high serum FSH concentrations.