When couples struggle with fertility, the focus is often on ovaries, fallopian tubes, or sperm. But there’s another quiet player in the background the thyroid gland. Many women (and men) get basic thyroid tests when investigating fertility. If the results fall within the “normal range,” they are often told everything is fine. However, newer research shows that even thyroid function that is “technically normal” can still hurt fertility, especially if thyroid-stimulating hormone (TSH) levels are not at their optimal point for pregnancy.
What Does the Thyroid Have to Do with Fertility?
The thyroid, a small butterfly-shaped gland in the neck, produces hormones that control metabolism, energy, and body temperature. But thyroid hormones also regulate the menstrual cycle, support egg development, and help maintain a healthy pregnancy.
If the thyroid is underactive (hypothyroidism), even slightly, it can lead to:
- Irregular periods
- Anovulation (failure to release an egg)
- Increased miscarriage risk
- Poor embryo implantation
Why “Normal” Isn’t Always Good Enough
The usual TSH reference range for the general population is about 0.4 to 4.0 mIU/L. But when it comes to fertility, experts now recommend that TSH be kept below 2.5 mIU/L, especially in women trying to conceive or undergoing IVF. This means a woman with a TSH of 3.5 technically “normal” could still have a harder time getting pregnant or face higher miscarriage risk.
Women with this mild dysfunction often:
- Feel normal
- Have regular periods
- Show no obvious symptoms
Yet, multiple studies show that subclinical hypothyroidism increases infertility risk by about 2-3 times. Even if thyroid hormone levels are currently normal, women with thyroid antibodies (like TPOAb) may face:
- Higher miscarriage rates
- Reduced IVF success rates
This happens because the immune system’s attack on the thyroid can spill over and affect the reproductive system.
Diagnosis and Treatment
- Test more deeply: Always check TSH, free T4, and thyroid antibodies (TPOAb, TgAb). Target lower TSH: Many fertility specialists aim for TSH <2.5 mIU/L before pregnancy.
- Simple treatment: Low dose levothyroxine (thyroid hormone replacement) can normalize TSH and improve outcomes. The treatment is usually safe, cheap, and straightforward yet many women are left untreated because their TSH is considered “normal” under outdated guidelines.
Conclusion
Your thyroid might be tiny, but its impact on fertility is huge. “Normal” lab results don’t always mean “optimal” when it comes to getting pregnant and staying pregnant. If you’re facing unexplained infertility, repeated miscarriages, or preparing for IVF, don’t settle for a basic thyroid check. Ask your doctor to dig deeper, and make sure your TSH is optimized not just normal. Sometimes, the difference between failure and success can be as small as a tiny shift in a hormone you can’t even feel.