Macro-FSH is a rare cause of inappropriately high FSH concentrations.

Summary: Assessment of hormone concentrations can be subjected to laboratory pitfalls. Macro-hormones are hormone-autoantibody complexes which are cleared slowly from circulation and cause a false elevation in hormones' concentrations. Macro-prolactin and macro-thyroid-stimulating hormone (TSH) are most frequently encountered while macro-follicle-stimulating hormone (FSH) has been rarely reported. We describe the case of a 30-year-old woman who had a gynaecological consultation due to failure in achieving pregnancy after 8 months of unprotected intercourse. She had regular menses, did not complain of climacteric symptoms and her medical history was unremarkable. Antral follicle count and anti-mullerian hormone concentrations were normal, and regular ovulation was documented. Unexpectedly, high early follicular phase FSH concentrations were confirmed on two occasions (57 and 51 IU/L), raising the suspicion of primary ovarian insufficiency. After excluding Turner's syndrome and autoimmune oophoritis, a laboratory artifact was hypothesized. Following polyethylene glycol precipitation, FSH levels dropped from 41.1 IU/L to 6.54 IU/L (recovery 16%) and the presence of macro-FSH was concluded. Laboratory interference can lead to misdiagnosis and unnecessary treatments. A laboratory artifact should be suspected when inconsistency exists between clinical presentation and laboratory results. Only five other cases of macro-FSH have been reported to date. Although macro-hormones generally have low biological activity and do not require treatment, the role of anti-FSH antibodies has been hypothesized in primary ovarian insufficiency and in vitro fertilization failure. Learning points: Hormone quantification is a cornerstone in the diagnostic workup of endocrine disorders, but it can be subjected to laboratory interferences which can lead to unnecessary investigations and inappropriate treatments. A laboratory artifact should be suspected when a discrepancy is observed between clinical presentation and laboratory results, when extremely unusual analyte concentrations are observed and when inconsistent results are obtained by different analytical methods. Macro-hormones are hormone-autoantibody complexes which are cleared slowly from circulation and cause a false elevation in hormone concentrations. Macro-prolactin and macro-TSH are most frequently encountered, while macro-FSH has been rarely reported. Macro-hormones can be detected by polyethylene glycol precipitation, gel filtration chromatography, or by using protein G or protein A columns. Although macro-hormones generally have low biological activity and do not require treatment, the role of anti-FSH antibodies has been hypothesized in primary ovarian insufficiency and in vitro fertilization failure.