Fertility and Thyroid Health: What Your TSH Level Should Be Before Trying to Conceive

Trying to get pregnant? If you’ve been trying for months without success, it’s easy to blame stress, timing, or bad luck. But there’s one hidden factor many doctors overlook: your thyroid. Specifically, your TSH level. For women planning pregnancy, TSH isn’t just a number on a lab report-it’s a silent gatekeeper to fertility and a healthy pregnancy.

Why TSH Matters More Than You Think

TSH, or thyroid-stimulating hormone, is the signal your brain sends to your thyroid to make hormones. When TSH is too high, your thyroid isn’t working hard enough. That’s hypothyroidism. Even if your TSH is in the "normal" range-say, 3.0 or 3.5-it might still be too high for conception.

The American Thyroid Association (ATA) says women trying to get pregnant should aim for a TSH level under 2.5 mIU/L. That’s not the same as the general population range (usually 0.4-4.5 mIU/L). Why the difference? Because your body needs more thyroid hormone during early pregnancy. Your baby can’t make its own thyroid hormones until about 12 weeks. Until then, it’s completely dependent on you.

A 2018 study of over 7,000 women found that those with unexplained infertility were nearly twice as likely to have TSH levels above 2.5 mIU/L compared to women who conceived easily. That’s not coincidence. Low thyroid hormone slows down ovulation, messes with egg quality, and can throw off your menstrual cycle. The Mayo Clinic puts it simply: "Low thyroid hormone can interfere with the release of an egg from your ovary."

What If You Have Hashimoto’s?

If you have Hashimoto’s thyroiditis-an autoimmune condition where your immune system attacks your thyroid-you’re at even higher risk. Studies show that women with Hashimoto’s need even tighter control before pregnancy. Experts recommend keeping TSH between 1.25 and 1.75 mIU/L, not just under 2.5. Why? Because your thyroid is already damaged. During pregnancy, your body’s demand for thyroid hormone jumps by 30-50%. If you’re already running low, you’ll crash faster.

One 2023 study from the American Thyroid Association found that women with Hashimoto’s who didn’t optimize their TSH before conception were far more likely to miscarry or deliver prematurely. And it’s not just about TSH. Your thyroid antibodies (TPOAb) matter too. Even if your TSH is normal, high antibodies can increase your risk of miscarriage by up to 50%. That’s why testing for both TSH and antibodies is critical if you have a history of autoimmune disease.

The Levothyroxine Trap

If you’re already on levothyroxine (the standard thyroid hormone replacement), you might think you’re covered. But here’s the catch: most women need a 25-50% dose increase as soon as they get pregnant. Yet, a 2019 study found that only 37% of women got their dose adjusted in time. Many waited until their first prenatal visit-by then, it might be too late.

Why does this happen? Doctors often don’t realize how quickly thyroid needs change. Your body starts producing more thyroid-binding proteins as soon as you conceive. That means more hormone gets bound up and becomes inactive. Your thyroid has to work harder, and if it’s already compromised, you’ll need more medication.

The fix? Don’t wait. If you’re planning pregnancy, talk to your doctor now. Get your TSH checked, and if you’re on levothyroxine, ask if your dose needs adjusting. Once you’re pregnant, retest every 4 weeks until you hit 20 weeks. Most women need a dose increase within the first 6 weeks.

And don’t use desiccated thyroid (like Armour Thyroid). It’s not stable enough for pregnancy. The InVia Fertility Center and other top reproductive clinics specifically warn against it because it lowers serum T4-the hormone your baby needs most.

Screening: Should Everyone Get Tested?

The American College of Obstetricians and Gynecologists (ACOG) doesn’t recommend universal TSH screening for all women trying to conceive. But the American Society for Reproductive Medicine (ASRM) does. And here’s why the debate matters: about 15-20% of women with infertility have abnormal TSH levels. That’s one in five. And thyroid dysfunction is one of the few fertility issues you can fix with a simple, cheap pill.

If you have any of these risk factors, get tested:

• History of thyroid disease or family history of it
• Unexplained infertility
• Irregular periods or very light bleeding
• Unexplained fatigue, weight gain, or hair loss
• Previous miscarriage
• Autoimmune conditions like type 1 diabetes or celiac disease

Even if you don’t have symptoms, if you’re over 30 and trying to conceive for more than 6 months, ask for a TSH test. It’s a simple blood draw. No fasting needed. Cost? Around $30-$60 out of pocket in the U.S. And if you’re on insurance, it’s often covered.

What About TSH Between 2.5 and 4.5?

This is where things get messy. Some studies say it’s fine. Others say it’s risky.

A 2017 study of women undergoing IUI found no difference in pregnancy or live birth rates between those with TSH under 2.5 and those between 2.5 and 4.5. That study got a lot of attention. But it had a small sample size and didn’t look at miscarriage rates.

Meanwhile, a massive 2018 study of over 180,000 women found that those with TSH between 2.5 and 4.5 had a 15% higher risk of miscarriage. That’s not trivial. And if you have thyroid antibodies? The risk jumps even higher.

Here’s the bottom line: if your TSH is 3.0 or above and you’re trying to conceive, don’t wait for symptoms. Ask your doctor to treat it. Levothyroxine is safe, cheap, and effective. A 2021 meta-analysis showed that treating subclinical hypothyroidism in infertile women reduced miscarriage rates by 45% and improved live birth rates by 36%.

How to Get Your TSH Right

If you’re planning pregnancy, here’s your step-by-step plan:

1. Get your TSH and TPOAb tested. Don’t skip the antibodies.
2. If your TSH is above 2.5, talk to your doctor about starting or adjusting levothyroxine.
3. If you have Hashimoto’s, aim for 1.25-1.75 mIU/L.
4. Take levothyroxine on an empty stomach with water. Wait at least 30 minutes before eating or taking other meds.
5. Avoid calcium, iron, or antacids within 4 hours of your dose-they block absorption.
6. Retest every 4 weeks until your TSH is stable under 2.5.
7. Once pregnant, retest every 4 weeks until week 20. Dose will likely need increasing.

The British Thyroid Foundation says most women need 3-6 months of stable TSH before trying to conceive. Don’t rush. Get it right.

The Cost of Waiting

Levothyroxine costs about $4-$10 a month. A single miscarriage? Around $7,200 in medical costs, lost wages, and emotional toll. A preterm birth? Can run over $50,000.

A 2021 study in the Journal of Managed Care & Specialty Pharmacy found that optimizing TSH before conception saves $1,850-$2,400 per pregnancy. That’s not just money-it’s time, hope, and heartache avoided.

And here’s something most people don’t know: the FDA required labs to use narrower reference ranges for pregnancy in 2017. But many commercial labs still print "normal" ranges up to 4.5. That means your doctor might see a TSH of 3.8 and say, "It’s fine." But it’s not fine for pregnancy.

What’s Next?

New research is coming. The NIH is running a trial (NCT03856002) to see if personalized TSH targets-based on your thyroid reserve and antibody levels-work better than the one-size-fits-all 2.5 target. Early data suggests yes. The European Thyroid Association already recommends even lower targets in the first weeks of pregnancy: under 1.8 mIU/L for weeks 1-4.

But you don’t need to wait for guidelines to change. Right now, the best advice is clear: if you’re trying to conceive, get your TSH checked. If it’s over 2.5, treat it. It’s one of the simplest, most effective things you can do for your fertility-and your future baby’s brain development.

Final Thought

Thyroid health isn’t glamorous. No one posts about it on Instagram. But it’s one of the most powerful levers you have for improving your chances of a healthy pregnancy. Don’t let a simple blood test slip through the cracks. Your future child’s brain is developing from day one. Make sure it has the fuel it needs.