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

If you’re trying to get pregnant and haven’t had success, your thyroid might be the missing piece. It’s not just about ovulation or sperm count-your thyroid hormone levels, especially TSH, play a huge role in whether conception happens and whether a pregnancy sticks. Yet, most women aren’t tested for this until after they’ve already been trying for months-or even years.

Why TSH Matters Before You Get Pregnant

TSH, or thyroid-stimulating hormone, is the signal your brain sends to your thyroid to make more hormones. When TSH is too high, your thyroid isn’t working hard enough. Even a small imbalance can mess with ovulation, hormone cycles, and the lining of your uterus. A 2018 study found that women 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 a fluke. Your thyroid doesn’t just regulate your metabolism-it’s deeply involved in reproductive health.

Many doctors still use a general TSH range of 0.4 to 4.0 or even 4.5 mIU/L. But that’s not the right target if you’re trying to get pregnant. The American Thyroid Association says clearly: if you’re planning pregnancy, your TSH should be below 2.5 mIU/L. Why? Because during the first 12 weeks of pregnancy, your baby relies entirely on your thyroid hormones. If your levels are even slightly low before conception, your baby’s brain development could be at risk.

The 2.5 mIU/L Rule-Where It Comes From

The 2.5 mIU/L cutoff didn’t come out of nowhere. It’s based on decades of research showing that women with TSH levels above this threshold-still technically "normal" by general standards-have higher rates of miscarriage, longer time to conception, and lower live birth rates. A large study of over 180,000 women found that those with TSH between 2.5 and 4.5 mIU/L had a 15% higher risk of pregnancy loss than those under 2.5. Another study showed that women with TSH over 2.5 took nearly twice as long to get pregnant compared to those with lower levels.

But here’s the catch: not every expert agrees. Some studies, including one from 2017, found no difference in pregnancy rates between women with TSH under 2.5 and those between 2.5 and 4.5. That’s why some doctors hesitate to treat borderline cases. The truth? It’s not black and white. If you have thyroid antibodies (like in Hashimoto’s), even a TSH of 2.6 might be too high. If you’re otherwise healthy with no autoimmune issues, you might be fine with a TSH of 2.4. But if you’re trying to conceive, aiming for under 2.5 is the safest bet.

What If You Have Hashimoto’s?

Hashimoto’s thyroiditis is the most common cause of hypothyroidism in women of childbearing age. If you have it, your thyroid is already under attack by your immune system. Pregnancy puts even more stress on it. Your body needs 30-50% more thyroid hormone in early pregnancy-and if you’re already barely keeping up, you’ll fall behind fast.

For women with Hashimoto’s, experts recommend getting TSH even lower before conception: between 1.25 and 1.75 mIU/L. Why? Because your thyroid can’t ramp up production quickly enough once you’re pregnant. If your TSH is already creeping up to 2.0 before you conceive, you’ll likely need a dose increase within weeks of a positive pregnancy test. Waiting until you’re pregnant to adjust your medication is too late.

And don’t rely on feeling fine. Many women with Hashimoto’s feel perfectly normal-even with TSH levels above 3.0. But feeling okay doesn’t mean your thyroid is doing its job for pregnancy. That’s why testing is non-negotiable.

Levothyroxine: The Only Safe Treatment

If your TSH is too high, levothyroxine is the standard treatment. It’s a synthetic form of T4, the main hormone your thyroid makes. It’s safe, effective, and the only thyroid medication recommended before and during pregnancy.

Don’t use desiccated thyroid (like Armour Thyroid). It contains both T4 and T3, but the T3 content is unpredictable and can cause swings in hormone levels. InVia Fertility Center and other top reproductive clinics specifically warn against it during preconception and pregnancy. Stick with levothyroxine-brands like Synthroid or Tirosint are preferred because they’re more consistent.

Once you start levothyroxine, you’ll need to get your TSH checked every 4 to 6 weeks until it’s stable. It takes about 6 weeks for your body to fully adjust to a new dose. Don’t rush it. And once you’re pregnant, your dose will likely need to go up by 25-50%. Many women don’t get this adjustment in time-studies show only about one in three women receive the right increase early enough. That’s why you need to talk to your doctor before you even miss a period.

How to Get Tested and What to Ask Your Doctor

Don’t wait for symptoms. If you’re trying to conceive, ask for a TSH test at your first fertility visit. The American Society for Reproductive Medicine recommends screening all women with infertility. Even if you’re not in a fertility clinic, your OB-GYN or GP should be willing to order it.

Ask for more than just TSH. Get your free T4 and thyroid antibodies (TPOAb) tested too. Antibodies tell you if your thyroid is being attacked by your immune system-even if your TSH is normal. If you have antibodies, your risk of miscarriage goes up, even without full-blown hypothyroidism. A 2023 review found that treating women with antibodies and normal TSH with levothyroxine reduced miscarriage risk by 45%.

And don’t be discouraged if your doctor says, "Your TSH is normal." Ask: "Is it normal for someone trying to conceive?" If they don’t know the 2.5 mIU/L target, it’s time to find someone who does. Reproductive endocrinologists and thyroid specialists are your best allies here.

How to Take Levothyroxine Right

Medication doesn’t work if you don’t take it properly. Levothyroxine needs to be taken on an empty stomach, at least 30 to 60 minutes before breakfast. Coffee, calcium supplements, iron, and even soy can block absorption. Wait at least four hours after taking your pill before consuming any of these.

Take it at the same time every day. Consistency matters. Many people find it easiest to take it first thing in the morning with a full glass of water. Don’t skip doses-even if you feel fine. Thyroid hormone levels build up slowly, and missing doses can push your TSH back up.

Keep a log of your doses and test results. Bring it to every appointment. This helps your doctor spot trends and make smarter adjustments.

What About Cost and Access?

Levothyroxine costs between $4 and $10 a month at most pharmacies. That’s less than your daily coffee. The real cost? Not treating it. A single miscarriage can cost upwards of $7,200 in medical care, lost wages, and emotional toll. Preconception thyroid screening and treatment save an estimated $1,850 to $2,400 per pregnancy by preventing early losses and preterm births.

Insurance usually covers TSH tests and levothyroxine. If you’re uninsured, many pharmacies offer generic levothyroxine for under $10 a month. Don’t let cost stop you. This is one of the cheapest, most effective ways to improve your chances of a healthy pregnancy.

What’s Changing in 2025?

The guidelines are evolving. The European Thyroid Association now recommends even tighter targets: TSH under 1.8 mIU/L in the first four weeks after conception, then gradually rising to 2.8 mIU/L by week 12. Why? Because the embryo’s brain starts developing before you even know you’re pregnant. Every day counts.

A major NIH-funded trial (NCT03856002) is wrapping up in late 2024 and may soon show that personalized TSH targets-based on antibody status and thyroid reserve-work better than a one-size-fits-all 2.5 cutoff. But until those results are published, sticking to under 2.5 is still the gold standard.

And here’s the bottom line: if you’re trying to conceive and haven’t been tested for thyroid issues, you’re not alone-but you’re also not doing everything you can. Thyroid dysfunction is one of the most treatable causes of infertility. Fix your TSH, and you might just fix your path to parenthood.