If you ask most men what drives fertility, testosterone is the first answer. And it matters-no question. But testosterone doesn't work alone. It's part of a signaling chain that starts in the brain and ends in the testicles. Without the other hormones in that chain, testosterone levels could be normal and sperm production could still stall.
Here is what the research shows about the hormones that actually orchestrate male fertility.
Follicle-Stimulating Hormone (FSH): The Sperm Production Director
FSH is produced in the pituitary gland, a pea-sized structure at the base of your brain. Its job in men is straightforward: tell the Sertoli cells in the testicles to support sperm production.
Think of Sertoli cells as the nursery. They nourish developing sperm cells, regulate the environment inside the seminiferous tubules (the tubes where sperm are made), and ensure that immature sperm mature properly. Without FSH signaling, those Sertoli cells don't get the message. Sperm count drops. Sperm morphology (shape) suffers.
A 2020 study in Human Reproduction Update reviewed data from men with isolated FSH deficiency. The consistent finding: severely reduced sperm counts, often below 5 million per milliliter (normal is 15 million or higher). When these men received FSH therapy, sperm counts increased significantly in about 60% of cases.
The practical takeaway: if a semen analysis shows low sperm count or poor morphology, FSH levels should be checked. Low FSH suggests the pituitary isn't sending the right signal. High FSH often means the testicles aren't responding, which points to a different issue.
Luteinizing Hormone (LH): The Testosterone Trigger
LH is FSH's partner. It travels from the same pituitary gland to the Leydig cells in the testicles. Its message: produce testosterone.
Without LH, Leydig cells go quiet. Testosterone production drops. And since intratesticular testosterone (the testosterone concentration inside the testicles) needs to be roughly 50 to 100 times higher than blood testosterone for normal sperm production, even a small drop in LH can have outsized effects.
One 2018 study in The Journal of Clinical Endocrinology & Metabolism measured LH and intratesticular testosterone in men with infertility. Men with low LH had intratesticular testosterone levels 70% lower than men with normal LH. Sperm production was correspondingly poor.
When doctors evaluate male infertility, they almost always check LH and FSH together. The ratio between them tells a story. High LH with normal testosterone suggests the testicles are resistant. Low LH with low testosterone suggests the pituitary is underactive.
Prolactin: The Overlooked Disruptor
Prolactin gets talked about mostly in the context of breastfeeding. But men produce it too, and when levels climb too high, fertility suffers.
Prolactin suppresses the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH is the master switch that tells the pituitary to release LH and FSH. So when prolactin is elevated, the entire chain gets muted. Testosterone drops. Sperm production drops.
A 2022 analysis in Fertility and Sterility looked at 450 men presenting with infertility. About 4% had hyperprolactinemia (elevated prolactin). Among those men, 70% had low testosterone and 55% had low sperm counts. Treating the underlying cause-often a small pituitary tumor or medication side effect-normalized prolactin in most cases, and fertility improved.
Prolactin is easy to test. It's often overlooked. If you're dealing with low libido, low testosterone, or unexplained infertility, ask your doctor to include it in the blood panel.
Thyroid Hormones (T3 and T4): The Metabolic Regulators
The thyroid gland produces hormones that control metabolic rate across every tissue in the body. That includes the testicles.
Thyroid hormone receptors are present on Sertoli cells and Leydig cells. When thyroid function is off, sperm production suffers. Both hypothyroidism (too little thyroid hormone) and hyperthyroidism (too much) have been linked to reduced sperm quality.
A 2019 study in Andrology compared semen parameters in men with untreated hypothyroidism against healthy controls. The hypothyroid group had significantly lower sperm motility (movement) and higher DNA fragmentation (damage to the genetic material inside sperm). After three months of thyroid hormone replacement, motility improved by an average of 18%.
The mechanism isn't fully understood, but it appears that thyroid hormones influence the energy metabolism of sperm cells and the environment inside the seminiferous tubules.
Thyroid testing (TSH, T3, T4) should be part of any comprehensive fertility workup, especially if there are other symptoms like fatigue, weight changes, or temperature sensitivity.
Estradiol: The Balance Keeper
Men produce estrogen too. The enzyme aromatase converts some testosterone into estradiol (the primary form of estrogen). That conversion happens in fat tissue, the brain, and the testicles themselves.
Estradiol is not the enemy. In the right range, it supports libido, bone density, and brain function. But when estradiol gets too high relative to testosterone, it signals the pituitary to reduce LH production. That creates a feedback loop: more estradiol means less LH, which means less testosterone, which means poorer sperm production.
A 2021 paper in The Journal of Steroid Biochemistry and Molecular Biology examined men with infertility and found that those with elevated estradiol-to-testosterone ratios had significantly lower sperm concentrations. The average sperm count in the high-estradiol group was 9 million per milliliter, compared to 22 million in the balanced hormone group.
What drives high estradiol? Excess body fat (especially abdominal fat) is the most common cause. Fat tissue contains aromatase. More fat means more conversion of testosterone to estradiol. Weight loss of 5-10% of body weight has been shown to lower estradiol and improve testosterone ratios in overweight men.
Inhibin B: The Direct Feedback Signal
Inhibin B is produced by the Sertoli cells. It tells the pituitary to reduce FSH production when sperm production is adequate. It's a feedback mechanism: when the testicles are working well, they release inhibin B, and the brain stops pushing so hard.
Low inhibin B means the testicles aren't producing enough sperm, and the brain keeps sending high FSH in response. This is why high FSH with low inhibin B is a classic pattern for testicular failure.
A 2020 study in Andrology found that inhibin B levels correlate more closely with sperm count than FSH does in some populations. The authors suggested that inhibin B could be a better marker for assessing testicular function in men with borderline semen analyses.
Inhibin B is not part of a standard blood panel. You have to ask for it. If you've had a semen analysis showing low sperm count and your FSH is normal, inhibin B might reveal what's really happening in the testicles.
Cortisol: The Stress Hormone That Interferes
Cortisol is produced by the adrenal glands in response to stress. Acute stress is fine. Chronic stress is where the problems start.
High cortisol suppresses GnRH production in the hypothalamus. That reduces LH and FSH release. Testosterone drops. Sperm production drops. It's the same mechanism as prolactin elevation, just from a different source.
A 2018 study in Psychoneuroendocrinology measured cortisol and semen parameters in 200 men over a year. Men with the highest cortisol levels had 35% lower sperm concentrations and 30% lower motility compared to men with normal cortisol. The relationship held even after controlling for age, BMI, and smoking.
Chronic stress management isn't soft advice. It has measurable effects on hormone levels. Sleep quality, exercise volume, and daily recovery practices all influence cortisol. If your lifestyle keeps cortisol high for months at a time, your fertility profile will reflect it.
What This Means for You
If you're trying to conceive and your semen analysis comes back less than ideal, testosterone is only part of the picture. A full hormone panel should include:
- LH and FSH (to assess pituitary signaling)
- Prolactin (to rule out suppression)
- TSH and free T3/T4 (to check thyroid function)
- Estradiol (to check the balance)
- Inhibin B (for direct testicular feedback)
- Cortisol