FSH Levels in Men with One Testicle
Men with a solitary testis typically have elevated FSH levels compared to men with bilateral testes, but FSH can still be within or near the normal range depending on the remaining testis's compensatory capacity. 1
Understanding the Compensatory Response
When a man has only one testicle, the remaining testis undergoes compensatory hypertrophy to maintain hormonal and reproductive function. However, this compensation is incomplete at the pituitary level:
- Men with a solitary testis demonstrate "compensated pituitary strain" with significantly higher FSH levels compared to men with bilateral testes who have identical sperm parameters. 1
- In a matched analysis controlling for sperm concentration and motility, men with one testis had median FSH of 13.9 mIU/ml (IQR 5.8-24.2) versus 5.0 mIU/ml (3.4-9.8) in men with two testes. 1
- Even in prepuberty, boys with unilateral testicular loss show significantly higher mean basal FSH levels than normal boys, with this elevation persisting throughout pubertal development. 2
Clinical Implications for Fertility
The elevated FSH in men with one testis reflects increased pituitary drive to maintain spermatogenesis, but does not necessarily indicate complete testicular failure:
- Men with a solitary testis can maintain sperm production at FSH levels that would indicate azoospermia in men with bilateral testes. Sperm have been documented in ejaculates at FSH levels as high as 54.6 mIU/ml in men with one testis, whereas men with two testes were uniformly azoospermic above 25.0 mIU/ml. 1
- Testosterone levels remain similar between men with one versus two testes (median 381 vs 387 ng/dl), indicating adequate Leydig cell compensation. 1
- However, long-term outcomes show that compensatory hypertrophy does not fully prevent testicular insufficiency—among boys with unilateral testicular loss followed to adulthood, 5 of 7 semen analyses showed oligospermia and 2 showed azoospermia. 2
Hormonal Pattern and Hypogonadism Risk
- The prevalence of compensated hypogonadism (elevated LH with normal testosterone) is significantly higher in men with a solitary testis (28%) compared to men with bilateral testes (8.2%). 1
- In pubertal stage 5, mean testosterone levels and response to hCG stimulation are significantly lower than normal in boys with unilateral testicular loss. 2
- This compensated state may represent a precursor to eventual pituitary "burnout" and declining testicular function over time, though longitudinal data are limited. 1
Diagnostic Approach
When evaluating a man with one testicle:
- Expect FSH to be elevated (often 10-25 mIU/ml range) even with preserved spermatogenesis—this does not automatically indicate non-obstructive azoospermia. 1
- Measure LH and testosterone to assess for compensated hypogonadism, which is common in this population. 1
- Semen analysis is essential, as FSH levels alone cannot predict fertility status—men with one testis maintain sperm production at FSH levels that would suggest severe dysfunction in men with two testes. 1
- Inhibin B levels may provide additional information about Sertoli cell function, particularly when FSH is in the 5-10 IU/L range. 3
Important Caveats
- The "normal" FSH reference range (typically 1.4-18.1 IU/L) does not account for the compensatory physiology in men with one testis—values of 10-20 mIU/ml may represent appropriate compensation rather than pathology. 4, 1
- FSH levels show natural variation due to pulsatile secretion, so single measurements should be confirmed if clinical decisions depend on the result. 5
- Never prescribe exogenous testosterone to men with one testis who desire fertility, as it will suppress the already-strained pituitary axis and potentially cause azoospermia. 5