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

When you're trying to get pregnant, you might check your ovulation calendar, track your cycle, or start taking prenatal vitamins. But how many of you know to check your TSH levels first? Thyroid-Stimulating Hormone isn't something most people think about when planning a baby-but it could be the missing piece in your fertility puzzle.

Why TSH Matters Before You Get Pregnant

Your thyroid doesn't just control your energy or weight. It plays a direct role in ovulation, egg quality, and early embryo development. Even mild thyroid problems-ones that don't cause obvious symptoms-can make it harder to get pregnant or increase your chance of miscarriage.

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. That’s not a fluke. The American Thyroid Association (ATA) set a clear target: if you're trying to conceive and have hypothyroidism, your TSH should be below 2.5 mIU/L. This isn't just a suggestion-it's based on data from tens of thousands of pregnancies showing better outcomes when TSH is kept low before conception.

What’s the Right TSH Level for Preconception?

Here’s where things get tricky. Most labs list a normal TSH range as 0.4 to 4.0 or even 4.5 mIU/L. But that’s for the general population-not women trying to get pregnant. For preconception care, the goal is tighter.

• For women with diagnosed hypothyroidism: Aim for TSH under 2.5 mIU/L before conception.
• For women with Hashimoto’s thyroiditis: Many experts recommend an even lower target-between 1.25 and 1.75 mIU/L-because your thyroid has to work harder once you're pregnant.
• For women with thyroid antibodies but normal TSH: Even if your TSH is under 2.5, having antibodies raises your miscarriage risk. Studies show levothyroxine can cut that risk by almost half.

The reason? During the first 10 weeks of pregnancy, the baby relies entirely on your thyroid hormone. If your levels dip too low, even briefly, it can affect brain development and increase the chance of losing the pregnancy.

Why 2.5? The Science Behind the Number

You might hear conflicting opinions. Some doctors say, “If your TSH is under 4.0, you’re fine.” But that’s based on outdated thinking.

A 2010 analysis by the ATA reviewed over 100 studies and found that women with TSH levels between 2.5 and 4.5 had higher rates of miscarriage, preterm birth, and low birth weight-even if they didn’t feel sick. One large study of 180,000 women showed a 15% higher risk of pregnancy loss if TSH was above 2.5.

But here’s the catch: not every study agrees. A 2017 trial looked at women undergoing IUI and found no difference in pregnancy rates between those with TSH under 2.5 versus those between 2.5 and 4.5. So why does the ATA still stand by 2.5?

Because real-world outcomes matter more than single trials. When you look at the full picture-miscarriage rates, preterm births, developmental risks-the evidence strongly supports keeping TSH low before conception. And if you're already on levothyroxine, there’s no downside to fine-tuning your dose.

What If You’re Already on Levothyroxine?

If you’re taking levothyroxine for hypothyroidism and you’re planning to get pregnant, you’re not done yet. Your dose will likely need to go up.

Once you conceive, your body’s demand for thyroid hormone jumps by 25% to 50%. Many women don’t realize this until it’s too late. A 2019 study found that less than 40% of women on levothyroxine had their dose adjusted after pregnancy was confirmed. That delay can be risky.

Here’s what to do:

1. Get your TSH checked before you stop using birth control.
2. If your TSH is above 2.5, talk to your doctor about increasing your levothyroxine dose.
3. Retest every 4 weeks until your TSH is stable under 2.5.
4. As soon as you get a positive pregnancy test, schedule another TSH test and expect your dose to increase again.

Don’t wait for symptoms. Fatigue, weight gain, or dry skin might not show up-even if your hormone levels are off.

What About Hashimoto’s or Thyroid Antibodies?

If you have Hashimoto’s, your immune system is attacking your thyroid. That doesn’t always mean your TSH is high-but it does mean your thyroid is under more stress.

Studies show that women with thyroid antibodies-even with normal TSH-have a higher risk of miscarriage. The good news? Taking levothyroxine can reduce that risk by 45%. The American Society for Reproductive Medicine (ASRM) recommends testing for antibodies if you’ve had unexplained infertility or previous miscarriages.

It’s not about whether you “feel sick.” It’s about protecting your baby’s development. Even if your TSH is normal, antibodies signal your thyroid is already struggling. Preconception treatment can make all the difference.

How to Get Your TSH Checked

This isn’t a routine test in most primary care visits. If you’re trying to conceive, you need to ask for it.

• Request a full thyroid panel: TSH, free T4, and thyroid peroxidase antibodies (TPOAb).
• Go to a reproductive endocrinologist or endocrinologist if you’re already in fertility care-they’ll know what to look for.
• Don’t rely on general lab ranges. Ask for the preconception target: under 2.5 mIU/L.

Some clinics screen everyone. Others wait until you’ve been trying for 6 months. Don’t wait. If you’re over 30, have a history of miscarriage, or have any symptoms like fatigue, hair loss, or cold intolerance, get tested now.

What About Diet, Supplements, and Natural Fixes?

You might hear about selenium, iodine, or ashwagandha helping thyroid health. While these can support overall function, they don’t replace levothyroxine if you have hypothyroidism.

Levothyroxine is synthetic T4-the exact hormone your body needs. It’s safe, effective, and well-studied in pregnancy. Natural thyroid extracts (like Armour Thyroid) are not recommended because they contain T3, which can cause unstable hormone levels and harm fetal development.

Also, avoid calcium, iron, or antacids within 4 hours of taking your thyroid medication. They block absorption. Take it on an empty stomach with water, at least 30 minutes before breakfast.

Cost and Practical Benefits

Levothyroxine costs between $4 and $10 a month. A single miscarriage can cost over $7,000 in medical care. Preconception thyroid optimization saves money, time, and heartache.

One study found that treating abnormal TSH before pregnancy saved $1,850 to $2,400 per pregnancy by reducing miscarriages and preterm births. That’s not just a medical win-it’s a personal one.

What’s Changing in 2025?

Guidelines are evolving. The European Thyroid Association now recommends even tighter targets in early pregnancy: TSH under 1.8 mIU/L in the first 4 weeks, then 2.2 by week 8. A major NIH trial is still underway to see if personalized TSH targets based on your thyroid reserve improve live birth rates.

The message is clear: thyroid health isn’t optional when you’re trying to conceive. It’s a foundational step-like folic acid, but often overlooked.

What to Do Next

If you’re planning a pregnancy:

• Ask your doctor for a TSH test-don’t wait for symptoms.
• If your TSH is above 2.5, work with an endocrinologist to adjust your dose.
• If you have Hashimoto’s or a history of miscarriage, get antibody testing.
• Once pregnant, retest TSH every 4 weeks until 20 weeks.
• Never stop or change your levothyroxine without medical advice.

Your thyroid doesn’t shout before it fails. But with a simple blood test and the right treatment, you can give your future baby the best start possible.