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 one of the most common and treatable causes of infertility is hiding in plain sight: your thyroid. Specifically, your TSH level. Many women don’t realize that even a slightly off thyroid can block ovulation, mess with hormone balance, and increase the risk of miscarriage - all before they even miss a period.

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. And when you’re trying to conceive, even a mild version can hurt your chances.

Research shows that women with unexplained infertility are nearly twice as likely to have a TSH level of 2.5 mIU/L or higher compared to women who conceive easily. In one study of over 7,000 women, those with TSH levels above 2.5 had a 15% higher chance of miscarriage. It’s not just about being diagnosed with thyroid disease - it’s about where your TSH sits within the normal range.

The old standard for "normal" TSH was up to 4.5 or even 5.0 mIU/L. But that range was based on the general population, not women planning pregnancy. For fertility, the goal isn’t just to be "normal." It’s to be optimal.

The Magic Number: Why 2.5 mIU/L Is the Target

The American Thyroid Association (ATA) recommends that women planning pregnancy keep their TSH below 2.5 mIU/L. This isn’t arbitrary. Studies tracking thousands of pregnancies found that women with TSH levels above this threshold had lower pregnancy rates and higher miscarriage risks - even if their TSH was still "within range."

Here’s the thing: during early pregnancy, your body can’t make enough thyroid hormone on its own. The baby relies on your thyroid for the first 10 to 12 weeks. If your thyroid is already working at its limit before conception, you won’t have enough reserve to support the baby’s brain development. That’s why experts say: don’t wait until you’re pregnant to fix it.

Women with Hashimoto’s thyroiditis - an autoimmune condition that attacks the thyroid - need to aim even lower. Many endocrinologists recommend TSH levels between 1.25 and 1.75 mIU/L before conception. Why? Because Hashimoto’s makes your thyroid more vulnerable to the increased demands of pregnancy. A TSH that’s "fine" now might spike dangerously high within weeks of conceiving.

What If Your TSH Is Between 2.5 and 4.5?

This is where things get messy. Some doctors say, "If it’s under 4.5, you’re fine." Others say, "You need treatment now." The truth? It depends.

If you have thyroid antibodies (like TPO antibodies), even a TSH of 3.0 increases your risk of miscarriage by about 50%. In that case, most experts agree: start levothyroxine. A 2023 review of clinical trials showed that treating women with antibodies and TSH above 2.5 reduced miscarriage rates by 45% and improved live birth rates by 36%.

But if you have no antibodies and no symptoms - no fatigue, no weight gain, no dry skin - the evidence is less clear. One study of women undergoing IUI found no difference in pregnancy rates between those with TSH under 2.5 and those between 2.5 and 4.5. That’s why some organizations, like ASRM, say we shouldn’t treat everyone with TSH between 2.5 and 4.5 - yet.

So what’s a woman to do? Get tested. If your TSH is above 2.5, ask for thyroid antibody tests. If they’re positive, treatment is likely worth it. If they’re negative and you’re otherwise healthy, monitor closely. But don’t wait until you’re pregnant to start.

How to Fix a High TSH Before Pregnancy

If your TSH is too high, the fix is usually simple: levothyroxine. It’s a synthetic version of the thyroid hormone your body should be making. It’s safe, well-studied, and inexpensive - often under $10 a month.

Here’s how it works:

1. Start with a low dose - usually 25 to 50 mcg daily - based on your weight and age.
2. Take it on an empty stomach with water, at least 30 to 60 minutes before eating or taking other meds.
3. Avoid calcium, iron, or multivitamins within 4 hours of your dose. They block absorption.
4. Get your TSH checked again in 4 to 6 weeks. Dose adjustments are made in 12.5 to 25 mcg increments.
5. Keep testing every 4 to 6 weeks until your TSH stays below 2.5 for two consecutive tests.

Don’t use desiccated thyroid (like Armour Thyroid) if you’re trying to conceive. Studies show it lowers T4 levels - the hormone your baby needs most - and can make things worse.

What Happens After You Get Pregnant?

Many women think once their TSH is under control, they’re done. Not even close.

Once you’re pregnant, your body’s demand for thyroid hormone jumps 25% to 50%. Most women need a higher dose of levothyroxine within the first 4 to 6 weeks. But here’s the scary part: a 2019 study found only 37% of women received a dose increase in time. That means two out of three women are running on empty during early pregnancy.

That’s why you need to:

• Tell your OB or midwife you’re on thyroid medication.
• Get your TSH tested as soon as you confirm pregnancy - don’t wait for your first appointment.
• Check TSH every 4 weeks until week 20, then every 6 to 8 weeks.

Don’t assume your pre-pregnancy dose is enough. Your body is changing. Your hormone needs are changing. Your thyroid needs to keep up.

Who Should Get Tested?

You don’t need to be diagnosed with thyroid disease to need this test. The American Society for Reproductive Medicine (ASRM) recommends screening TSH for all women seeking fertility help. But here’s who else should ask:

• Women over 35 trying to conceive
• Those with a family history of thyroid disease
• Anyone with unexplained fatigue, weight gain, hair loss, or irregular periods
• Women who’ve had a miscarriage before
• Those with type 1 diabetes, lupus, or other autoimmune conditions

Thyroid problems are silent. You can have them and feel fine. But your fertility doesn’t care how you feel - it only cares about your hormone levels.

The Cost of Waiting

Levothyroxine costs about $4 to $10 a month. A single miscarriage? That can cost $7,200 in medical care, lost wages, and emotional toll. Preconception TSH screening and treatment saves an average of $1,850 to $2,400 per pregnancy by reducing miscarriages and preterm births.

And it’s not just about money. One study estimates that if every woman planning pregnancy had her TSH checked and optimized, the U.S. could prevent 65,000 to 80,000 miscarriages every year. That’s more than the population of a mid-sized city.

What’s Next? New Guidelines Coming

The American Thyroid Association is updating its guidelines in 2025, and early drafts suggest even tighter targets. The European Thyroid Association already recommends TSH below 1.8 mIU/L in the first 4 weeks after conception. A major NIH trial is now testing whether personalized TSH goals - based on your thyroid reserve and antibody status - lead to better outcomes than a one-size-fits-all 2.5 target.

For now, the safest, simplest, and most proven step you can take before trying to conceive is this: get your TSH checked. If it’s above 2.5, talk to your doctor about levothyroxine. Don’t wait. Don’t assume you’re fine. Your thyroid is one of the easiest things to fix - and one of the most powerful levers for fertility.