Hypogonadism is a clinical condition in which the testes do not produce adequate testosterone. The diagnosis requires two components: consistently low testosterone on morning blood tests, and symptoms attributable to that deficiency. Low testosterone on a lab test without symptoms is not hypogonadism by clinical definition. The Endocrine Society diagnostic guidelines are explicit on this point.

The most clinically important classification distinguishes primary from secondary hypogonadism, because the location of the problem determines what treatment options are appropriate.

Primary Hypogonadism

Primary hypogonadism means the failure is in the testes themselves. The brain sends the correct hormonal signals, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are elevated, but the testes cannot respond adequately to produce testosterone.

Elevated LH and FSH in the presence of low testosterone is the biochemical signature of primary hypogonadism. The pituitary gland detects low testosterone and increases its LH and FSH output in an attempt to stimulate more production, but the testes are unable to respond.

Common causes of primary hypogonadism:

  • Klinefelter syndrome (47,XXY karyotype), the most common genetic cause, present in approximately 1 in 500 to 1 in 1,000 males
  • Orchitis (testicular inflammation) from mumps or other infections
  • Testicular torsion or injury
  • Cancer treatment (chemotherapy and radiation can permanently damage Leydig cells, the testosterone-producing cells)
  • Varicocele, enlarged veins in the scrotum that increase testicular temperature and impair function

Secondary Hypogonadism

Secondary hypogonadism means the problem originates in the hypothalamus or pituitary gland, not the testes themselves. The brain fails to produce adequate LH and FSH, so the testes receive insufficient stimulation and produce less testosterone.

Low or inappropriately normal LH and FSH in the presence of low testosterone points toward secondary hypogonadism. The testes themselves are capable of producing testosterone if appropriately stimulated, but the signal is not reaching them.

Common causes of secondary hypogonadism:

  • Obesity, excess adipose tissue increases aromatase activity (converting testosterone to estrogen) and impairs hypothalamic GnRH secretion
  • Opioid use, opioids suppress GnRH release, a well-documented effect that produces secondary hypogonadism in a significant proportion of long-term opioid users
  • Pituitary adenomas, benign tumors in the pituitary that compress surrounding tissue
  • Traumatic brain injury
  • Hyperprolactinemia, elevated prolactin (from a pituitary prolactinoma or certain medications) suppresses GnRH
  • Anabolic steroid use, exogenous testosterone suppresses the HPG axis, causing secondary hypogonadism that persists after stopping steroids

Why the Distinction Matters for Treatment

Testosterone replacement therapy (TRT) works for both primary and secondary hypogonadism. External testosterone supplementation restores testosterone levels regardless of whether the problem is testicular or central.

However, TRT suppresses the HPG axis. When exogenous testosterone is detected by the hypothalamus and pituitary, they reduce LH and FSH output further. This leads to testicular atrophy and suppression of sperm production. For men with secondary hypogonadism who want to preserve fertility, TRT is not the right first-line approach.

Clomiphene citrate (an estrogen receptor modulator) works specifically for secondary hypogonadism by blocking estrogen’s negative feedback on the hypothalamus, causing the pituitary to produce more LH and FSH, which then stimulates the testes to produce more testosterone naturally. Clomiphene requires a functioning hypothalamic-pituitary axis and functioning testes, it is inappropriate for primary hypogonadism where the testes cannot respond. More detail on this option is in Clomiphene for Low Testosterone: An Alternative to TRT.

HCG (human chorionic gonadotropin) mimics LH and directly stimulates the Leydig cells in the testes. It is used to maintain testicular function and sperm production in men on TRT, and as an alternative or adjunct to TRT in secondary hypogonadism. Coverage of this is in the article on HCG on TRT.

Diagnosing Hypogonadism

A complete diagnostic workup includes:

  1. Total testosterone: Two morning measurements (7-10 am) on separate days, both below 300 ng/dL, establish the biochemical component
  2. LH and FSH: Distinguish primary from secondary
  3. Prolactin: Elevated prolactin suggests a pituitary prolactinoma
  4. SHBG: High SHBG reduces bioavailable testosterone and provides context for the total testosterone number
  5. Complete blood count: Testosterone replacement increases red blood cell production; baseline is needed

For a full explanation of how testosterone reference ranges are interpreted by decade, see Testosterone Levels by Age: What the Numbers Actually Mean.