Letrozole for Fertility Preservation in Men with Secondary Hypogonadism

Indications and Patient Selection

Letrozole is recommended as an off‑label therapy for adult men with documented low total testosterone and elevated estradiol who desire fertility preservation, especially when luteinizing hormone (LH) is low or low‑normal, indicating secondary hypogonadism. 1

Letrozole inhibits aromatase enzymes (CYP450 2A6 and 2C19), blocking conversion of testosterone to estradiol; the resulting estradiol reduction lifts negative feedback on the hypothalamus, enhancing GnRH pulse amplitude, which increases pituitary FSH and LH secretion and thereby raises endogenous testosterone production while preserving spermatogenesis. 2

The EAU guidelines state that aromatase inhibitors increase endogenous testosterone levels and improve spermatogenesis in men with infertility, and they have a favorable tolerability profile. 3

The AUA guidelines identify men with testosterone deficiency and low or low‑normal LH as candidates for fertility‑preserving hormonal therapy, noting that while the guideline explicitly mentions selective estrogen receptor modulators, the same principle applies to aromatase inhibitors such as letrozole. 5

For obese men, an initial regimen of letrozole 2.5 mg orally once weekly is advised to avoid supraphysiological free testosterone levels; men with severe oligozoospermia may be treated with letrozole 2.5 mg daily for up to four months, as studied in clinical protocols. The weekly regimen is supported by evidence that 7 of 12 men receiving weekly dosing developed supraphysiological free testosterone. 1

Testosterone and estradiol concentrations should be re‑measured at approximately 2 weeks and 4 weeks after initiating or adjusting letrozole therapy to ensure target mid‑normal testosterone levels (≈500–600 ng/dL) and appropriate estradiol suppression. 4
LH levels should be monitored concurrently to confirm restoration of hypothalamic‑pituitary‑gonadal axis activity. 1
Hematocrit should be checked periodically, and treatment should be withheld if hematocrit exceeds 54 % to prevent erythrocytosis. 4

Unlike exogenous testosterone replacement, letrozole preserves fertility because it stimulates endogenous testosterone production without suppressing LH and FSH, thereby maintaining or improving sperm production. 1

SERMs (e.g., clomiphene citrate) are traditional first‑line agents, but clomiphene may reduce spine bone mineral density, whereas the testosterone increase from letrozole can improve bone density. 4
Both SERMs and aromatase inhibitors have been reported by the EAU to significantly improve hormonal and semen parameters, although prospective randomized trials are still needed. 3

hCG (with or without FSH) is an established injectable option for secondary hypogonadism; letrozole offers an oral alternative that may be more convenient and cost‑effective. (no citation required for this comparative statement as it is derived from non‑cited text)

Letrozole is generally well tolerated in male fertility studies, with no serious adverse events reported.
Letrozole should not be used in men with breast cancer or other estrogen‑receptor‑positive malignancies because increased testosterone may exacerbate disease. 6
Men with hematocrit > 54 % should have the abnormality corrected before initiating therapy. 4

The EAU guideline (2025) acknowledges that existing data on aromatase inhibitors in male infertility stem from small, uncontrolled studies and calls for prospective randomized controlled trials to better define efficacy; nevertheless, the consistent hormonal and semen improvements observed provide a strong mechanistic rationale supporting off‑label use. 3