# Isolated Follicle-Stimulating Hormone (FSH) Deficiency in Male Sex: A Case Report

## TL;DR

A 30-year-old man with low FSH levels and gynecomastia may have isolated FSH deficiency, a rare condition affecting male fertility.

## Contribution

This case report highlights IFD as a possible cause of male hypogonadism symptoms despite normal testosterone levels.

## Key findings

- The patient had decreased FSH levels and inadequate FSH response to GnRH stimulation.
- Genetic testing for FSHB mutations and whole exome sequencing found no pathogenic variants.
- IFD should be considered in men with hypogonadism-like symptoms and normal testosterone.

## Abstract

Follicle-stimulating hormone (FSH) is a glycoprotein hormone produced in the anterior pituitary, essential in the regulation of gonadal functions. Isolated FSH deficiency (IFD) is a rare inherited disorder, usually caused by β-subunit alterations. In men, it is frequently detected during infertility evaluation, commonly associated with spermatogenesis impairment and testicular atrophy with normal testosterone levels.

We present the case of a 30-year-old male patient who was referred for breast pain and bilateral gynecomastia. Laboratory evaluation displayed primary hypothyroidism with positive thyroid autoantibodies and decreased FSH levels, with normal total testosterone and adequate luteinizing hormone (LH) levels. A gonadotropin-releasing hormone (GnRH) stimulation test demonstrated inadequate FSH response, highly suggestive of IFD. Genetic testing for FSHB mutations was negative, and whole exome sequencing revealed no recognized pathogenic variants. Semen analysis was postponed by the patient’s choice.

Although rare, IFD should be considered when evaluating patients with symptoms suggestive of hypogonadism. The overall prevalence of FSH-β mutations is unknown but probably underdiagnosed in male patients.