Key takeaways:
- Thyroid stimulating hormone, or TSH, is not part of the reproductive system, but a meta-analysis of 2,239 men found that subclinical hypothyroidism was associated with worse semen morphology and progressive motility (Bahreiny et al., Reproductive and Developmental Medicine, 2025).
- A high TSH usually signals an underactive thyroid and a low TSH usually signals an overactive one, and both directions have been linked to lower testosterone and abnormal semen parameters, not just one.
- Major guidelines do not recommend routine TSH screening for every subfertile man, only for men with erectile or ejaculatory problems or unexplained abnormal semen results (Poppe et al., European Thyroid Journal, 2021), so this is a targeted test, not a first-line one.
Most men have never had a reason to think about their thyroid until a routine blood panel flags TSH as high or low. It sounds unrelated to fertility. It is not. The thyroid gland sits in the neck, not the groin, but the hormones it produces reach every cell in the body, including the ones responsible for making testosterone and sperm. When thyroid function drifts too far in either direction, the effects on male fertility are real, measurable, and frequently missed because most fertility workups start and stop at FSH, LH, and testosterone.
This article explains what TSH actually measures, what a high or low result means for your thyroid and your fertility, and where the evidence is strong versus where it is still thin. TSH is a smaller piece of a male fertility hormone picture than testosterone or FSH, and we will be upfront about that scale throughout.
What does TSH actually measure, and why does it matter for fertility?
TSH does not come from the thyroid itself. It comes from the pituitary gland, the same small structure at the base of the brain that also produces FSH and LH, the hormones covered in earlier parts of this series. TSH is the pituitary's instruction to the thyroid gland, telling it how much thyroid hormone to produce. Thyroid hormone, mostly a form called T4 that gets converted in tissues to the more active T3, then governs metabolism in nearly every organ in the body.
The signaling chain works like this: the hypothalamus releases a hormone called TRH, which tells the pituitary to release TSH, which tells the thyroid gland to release T4 and T3. This is sometimes called the HPT axis, for hypothalamus, pituitary, thyroid, and it runs in parallel to the HPG axis, for hypothalamus, pituitary, gonads, that governs testosterone and sperm production and that we have covered in our FSH and LH articles. These two systems are run by the same pituitary gland and the same brain region, and they cross-talk with each other more than most men realize.
Thyroid hormone receptors have been found directly on Sertoli cells, the supporting cells in the testes that nurture developing sperm, and on Leydig cells, the cells that produce testosterone. T3 appears to help Sertoli cells mature properly during development and supports the machinery Leydig cells use to convert cholesterol into testosterone (Kumar et al., Indian Journal of Endocrinology and Metabolism, 2014). In plain terms, thyroid hormone is not just a general metabolism dial. It has a direct, physical job to do inside the testes.
What does a normal TSH level mean?
The standard reference range for TSH in adults is roughly 0.4 to 4.5 mIU/L, though the exact cutoffs vary slightly by lab and assay, and the upper limit tends to creep higher with age. Within that range, the thyroid is producing enough hormone to keep the HPT axis in balance without the pituitary needing to push harder or ease off.
A normal TSH result does not rule out every thyroid-related fertility concern, but it does rule out the two conditions this article focuses on: an underactive thyroid and an overactive one. If your TSH is normal and your FSH, LH, and testosterone results are the ones raising questions, thyroid function is unlikely to be the explanation, and the more useful next step is looking at those hormones directly, which our testosterone article covers in depth.
What does high TSH mean for your fertility?
A high TSH usually means the thyroid is underactive, a condition called hypothyroidism. This can feel counterintuitive at first. The reason TSH rises is that the pituitary is compensating: the thyroid is not making enough hormone, so the pituitary sends a louder and louder signal trying to get it to produce more. A high TSH is the pituitary shouting at an underactive thyroid, not the thyroid itself being overactive.
Hypothyroidism has been associated with lower total and free testosterone, lower sex hormone binding globulin (SHBG, a protein that carries testosterone through the blood), reduced sperm motility, abnormal sperm shape, and reduced semen volume (Sengupta et al., Life, 2022). A meta-analysis pooling 10 studies and 2,239 men with subclinical hypothyroidism, meaning a mildly elevated TSH with otherwise normal thyroid hormone levels, found consistent associations with worse semen morphology and progressive motility, alongside a smaller, not statistically significant trend toward lower semen volume (Bahreiny et al., Reproductive and Developmental Medicine, 2025). Hypothyroidism also raises a hormone called TRH, which as a side effect stimulates prolactin production, and elevated prolactin independently suppresses testosterone and sperm production, a mechanism our prolactin article covers in detail. This is one reason thyroid function and prolactin are often checked together.
Men with hypothyroidism also commonly report reduced libido, erectile dysfunction, and delayed ejaculation, which are hormonal and vascular effects of low thyroid hormone rather than a separate, unrelated problem. The encouraging part of this picture is that hypothyroidism is one of the more directly treatable hormonal contributors to male fertility problems. Correcting it with levothyroxine, a synthetic thyroid hormone taken as a daily pill under a doctor's supervision, has been associated with improvement in sperm morphology and motility in men who were hypothyroid to begin with (Sengupta et al., Life, 2022).
What does low TSH mean for your fertility?
A low TSH usually means the opposite problem: an overactive thyroid, a condition called hyperthyroidism. Here the thyroid is producing too much hormone on its own, so the pituitary pulls back on TSH because it does not need to push for more.
Hyperthyroidism raises SHBG, which paradoxically pushes total testosterone up while free testosterone, the portion actually available to tissues, can stay flat or even fall (Kumar et al., Indian Journal of Endocrinology and Metabolism, 2014). Excess thyroid hormone also increases the conversion of testosterone into estradiol, the form of estrogen men also produce, which we cover in our estradiol article. On semen parameters specifically, hyperthyroidism has been associated with reduced semen volume, lower sperm concentration, reduced motility, and a lower proportion of normally shaped sperm (Sengupta et al., Life, 2022). A dedicated meta-analysis of 748 men with subclinical hyperthyroidism, a mildly suppressed TSH with otherwise normal thyroid hormone levels, confirmed measurable effects on both reproductive hormones and semen quality compared to men with normal thyroid function (Bahreiny et al., Reproductive Sciences, 2024).
As with hypothyroidism, the fertility-relevant effects of hyperthyroidism have been shown to improve once thyroid hormone levels are brought back to normal, a state clinicians call euthyroid, using antithyroid medication, radioactive iodine, or surgery depending on the underlying cause (Sengupta et al., Life, 2022). None of these are decisions to make without an endocrinologist, and none of them are things a supplement can substitute for.
How should you read TSH alongside your other hormones?
TSH on its own tells you about thyroid function. It becomes more useful for fertility once you read it next to FSH, LH, and testosterone, because the combination tells you whether an abnormal reproductive hormone pattern has an explanation sitting one system over.
Low testosterone with a high TSH points toward hypothyroidism as a plausible contributor, and correcting the thyroid problem is the logical first step before considering testosterone-specific treatment. Low testosterone with a normal TSH means thyroid function is not the explanation, and the FSH and LH pattern described in our testosterone article becomes the more useful diagnostic thread to pull. Elevated prolactin alongside an abnormal TSH is a classic combination worth flagging to a clinician, since hypothyroidism is one of the more common, correctable causes of elevated prolactin. A normal TSH alongside abnormal semen parameters simply means you can set thyroid function aside and look elsewhere, which is a useful thing to rule out even when the answer is negative.
What we are more skeptical about
The honest picture here is more modest than a headline like "your thyroid could be ruining your fertility" would suggest. In the cross-sectional study most directly measuring TSH against semen parameters in a general fertility clinic population, 130 men, TSH explained less than 7% of the variation in sperm concentration and less than 3% of the variation in semen volume, and the relationship with concentration was not a simple straight line but a biphasic one, rising then falling around a TSH of roughly 1.2 mIU/L (Kakoulidis et al., Medical Sciences, 2022). That is a real signal, but a small one, and TSH alone is a poor predictor of any individual man's semen quality.
An earlier and larger study of 305 infertile men found no correlation at all between latent thyroid dysfunction and semen parameters, though it did find that men with elevated thyroid antibodies, a marker of autoimmune thyroid disease rather than TSH itself, were more likely to have abnormal sperm shape and reduced motility (Trummer et al., Fertility and Sterility, 2001). Findings like this are part of why the 2021 European Thyroid Association guideline explicitly recommends against routinely screening TSH in every man from a subfertile couple, reserving that test for men who already have erectile or ejaculatory dysfunction or an unexplained abnormal semen analysis, and it grades even that narrower recommendation as based on low quality evidence (Poppe et al., European Thyroid Journal, 2021). The same guideline is also clear that mild or subclinical thyroid dysfunction in a man should not delay fertility treatments like IVF or ICSI unless semen parameters are severely affected. We think that is the right level of caution: TSH is worth checking in the right context, not worth treating as a routine part of every fertility workup, and not something to self-diagnose from symptoms alone.
What can be done
If your TSH comes back abnormal, the path forward is medical, not nutritional. There is no supplement that safely or reliably corrects a genuinely underactive or overactive thyroid, and treating a real thyroid disorder with iodine, selenium, or any other over-the-counter product instead of an evaluated, monitored prescription is not something the evidence supports. An abnormal TSH result calls for confirmation with a repeat test and, typically, a free T4 measurement to clarify the picture, followed by treatment matched to the actual cause, whether that is levothyroxine for hypothyroidism or one of several options for hyperthyroidism, managed by a physician who can track your levels over time.
Your SwimScore hormone results give you FSH, LH, and testosterone, the pattern that tells a clinician whether a low testosterone problem sits in the testes themselves or further upstream. TSH is not part of that same panel, but it is an inexpensive, widely available blood test any doctor can add, and it is worth requesting specifically when your FSH, LH, and testosterone results do not add up to a clear story on their own, when prolactin is also elevated, or when you have symptoms like unexplained fatigue, weight change, or low libido alongside your fertility concerns. Correcting a genuine thyroid disorder takes time to show up in sperm, generally at least the 12 weeks it takes for a new batch of sperm to fully develop, so patience and a retest matter here as much as they do with any other hormonal intervention.
Our take
What we know: thyroid hormone acts directly on the cells in the testes that produce testosterone and support developing sperm, and both an underactive and an overactive thyroid have been associated with worse semen parameters and lower usable testosterone, not just one direction. What we don't know, and won't overstate, is how much TSH alone predicts any individual man's fertility outcome. The best available data puts that predictive power in the single digits, and one well-conducted older study found no relationship at all once you looked past thyroid antibodies specifically.
The practical takeaway is a narrow one, and we think that is appropriate given the evidence. TSH is not a hormone to test reflexively alongside FSH, LH, and testosterone. It becomes worth testing when those core results do not explain what you are seeing, when prolactin is elevated, or when you have symptoms that fit a thyroid problem. When TSH is genuinely abnormal, correcting it is one of the more directly treatable hormonal contributors to male fertility, with a clear, monitored medical path rather than a supplement aisle.
FAQ
Can an underactive thyroid cause male infertility?
It can contribute to it. Hypothyroidism has been associated with lower testosterone, reduced sperm motility, abnormal sperm shape, and reduced semen volume, and these changes have been shown to improve once thyroid hormone is corrected with levothyroxine (Sengupta et al., Life, 2022).
Can hyperthyroidism cause infertility in men?
Yes. An overactive thyroid raises SHBG and can lower the free testosterone available to tissues, and has been associated with reduced semen volume, lower sperm concentration, reduced motility, and abnormal sperm shape, with improvement seen once thyroid hormone levels return to normal (Sengupta et al., Life, 2022).
What is a normal TSH level for fertility?
The standard adult reference range is roughly 0.4 to 4.5 mIU/L, though exact cutoffs vary by lab. A normal TSH does not guarantee normal fertility, but it does rule out an underactive or overactive thyroid as the explanation for abnormal results.
Should every man getting a fertility workup have his TSH tested?
Not according to current guidelines. The European Thyroid Association recommends against routine TSH screening in men from subfertile couples, reserving it for men with erectile or ejaculatory dysfunction or an unexplained abnormal semen analysis (Poppe et al., European Thyroid Journal, 2021).
Does fixing a thyroid problem improve sperm quality?
In men who were genuinely hypothyroid or hyperthyroid, correcting thyroid hormone levels back to normal has been associated with improvement in sperm motility and morphology, though this takes time, generally at least the 12 weeks a new batch of sperm needs to develop (Sengupta et al., Life, 2022).
References
- Kumar A, Shekhar S, Dhole B. Thyroid and male reproduction. Indian Journal of Endocrinology and Metabolism. 2014;18(1):23-31. https://pmc.ncbi.nlm.nih.gov/articles/PMC3968728/
- Sengupta P, Dutta S, Karkada IR, Chinni SV. Endocrinopathies and Male Infertility. Life. 2022;12(1):10. https://pmc.ncbi.nlm.nih.gov/articles/PMC8779600/
- Kakoulidis I, et al. Study on the Interaction between Serum Thyrotropin and Semen Parameters in Men. Medical Sciences. 2022;10(2):22. https://pmc.ncbi.nlm.nih.gov/articles/PMC9036276/
- Bahreiny SS, Ahangarpour A, Aghaei M. Assessment of the effects of subclinical hypothyroidism on male reproductive parameters: a meta-analysis approach. Reproductive and Developmental Medicine. 2025;9(1):57-65. https://doi.org/10.1097/RD9.0000000000000111
- Bahreiny SS, et al. Meta-Analytical and Meta-Regression Evaluation of Subclinical Hyperthyroidism's Effect on Male Reproductive Health: Hormonal and Seminal Perspectives. Reproductive Sciences. 2024;31:2957-2971. https://doi.org/10.1007/s43032-024-01676-8
- Trummer H, Ramschak-Schwarzer S, Haas J, Habermann H, Pummer K, Leb G. Thyroid hormones and thyroid antibodies in infertile males. Fertility and Sterility. 2001;76(2):254-257. https://pubmed.ncbi.nlm.nih.gov/11476769/
- Poppe K, Bisschop P, Fugazzola L, Mintziori G, Unuane D, Weghofer A. 2021 European Thyroid Association Guideline on Thyroid Disorders prior to and during Assisted Reproduction. European Thyroid Journal. 2021;9(6):281-295. https://pmc.ncbi.nlm.nih.gov/articles/PMC7923920/
This is Part 6 of the SwimScore Hormone Series. Next up: SHBG, the carrier protein that determines how much of your testosterone is actually usable.
See the earlier hormone articles on FSH, LH, testosterone, prolactin, and estradiol.
Want the FSH, LH, and testosterone pattern that tells you whether a thyroid workup is worth pursuing? SwimScore Complete measures your full hormone panel alongside sperm concentration, motility, morphology, and DNA fragmentation, from home, processed in CLIA-certified labs.
Fertility clinics and urology practices: if a patient's hormone results do not add up without a thyroid explanation, see how SwimScore can provide a same-visit baseline panel on our clinic page.
This article is for general education and is not medical advice. Any suspected thyroid disorder should be evaluated and managed by a physician or endocrinologist.
SwimScore uses CLIA-certified labs for all semen analysis and hormone testing, assessed against WHO 6th Edition clinical thresholds.
