Should I get my thyroid checked before getting pregnant?

Why the thyroid matters so early

Your thyroid is a small gland at the front of your neck. It makes hormones that set your body's pace, and thyroid hormone is needed for a baby's brain and nervous system to develop.

Here's the part most people don't know: early in pregnancy, your baby can't make its own thyroid hormone yet. The fetus depends entirely on yours. And your thyroid can't make hormone without iodine. So in the first trimester, your thyroid and your iodine are your baby's thyroid and iodine.

A lot happens in that stretch. Counting from your last period, the brain, spinal cord and heart begin to develop around week 5, and by weeks 6 to 7 the brain has formed into five different areas, according to MedlinePlus. All of that early building happens while your baby is relying on your thyroid hormone.

That's why a check before pregnancy makes sense. By the time most people have their first prenatal visit, the early weeks are already under way.

The test: TSH, and how to read the idea behind it

The usual test is called TSH (thyroid-stimulating hormone). It's a simple blood draw. When your thyroid runs slow, your brain shouts louder at it, so TSH goes up. A high TSH usually means an underactive thyroid.

About 2.5% of pregnant women have a slightly high TSH (above 6), according to the American Thyroid Association. A slightly slow thyroid is easy to miss, because tiredness and weight gain are easy to blame on life, work or small children.

Your doctor will interpret your number. If you already take thyroid medicine, the questions shift. Ask what TSH they want you at before you conceive, and what you should do with your dose when you get a positive test. Have that conversation before you start trying, so there's a plan ready.

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What the studies found

A US study of untreated low thyroid. Researchers tested blood saved from 25,216 pregnant women and found 48 whose low thyroid had never been treated during that pregnancy. Untreated low thyroid was linked with children who averaged 7 points lower on IQ tests at ages 7 to 9 than children of matched mothers with normal thyroid levels. Eleven years later, 64% of the untreated mothers had confirmed hypothyroidism.

A large trial with an honest twist. In the CATS trial in the UK and Italy, 21,846 pregnant women were screened around week 12, and those with low thyroid function got thyroid medicine starting around week 13. Their children's IQ at age 3 was no different from children of untreated mothers: 99.2 versus 100.0.

One reading is that week 13 may be too late to make a difference. That's only a hypothesis. Either way, it argues for knowing your thyroid status before pregnancy, not after.

More isn't better. In a later look at the CATS children, those whose mothers were overtreated had a higher risk of conduct and hyperactivity traits at age 9. The goal is the right level, which is why dosing is your doctor's job.

The first study is observational, so it shows a link, not proof. Taken together, the message is calm and practical: check early, treat when needed, and aim for the right level.

Iodine: what your thyroid runs on

Your thyroid uses iodine to make its hormone. The WHO recommends 250 mcg of iodine a day in pregnancy and breastfeeding, compared with 150 mcg for other adults. The American Thyroid Association suggests women planning a pregnancy take a supplement with 150 mcg a day, ideally starting three months before.

Why early? Your thyroid stores iodine and seems to like a slow fill. In a study of 433 pregnant women in Sicily, women who used iodized salt for over two years before pregnancy were much less likely to have low thyroid hormone. Women who started a supplement only in early pregnancy were also protected, but many had a temporary jump in TSH.

Easy moves: use iodized salt for the salt you already add at home (sea salt and pink salt usually aren't iodized), keep dairy and seafood in your week if you eat them, and check that your prenatal lists iodine. More in iodine during pregnancy.

One caution: people with thyroid disease can react to iodine amounts that are fine for others. If you have a thyroid condition, ask your doctor about iodine before changing anything, and skip kelp supplements unless they say otherwise.

Putting it on your check-up list

Ask for TSH by name at your preconception visit, along with ferritin for iron and an A1c for blood sugar if you have risk factors. The full list is in what blood tests to get before trying to get pregnant. Useful lines to bring:

• “Can you check my TSH before I start trying?”
• “What level do you want before I conceive?”
• If you take thyroid medicine: “What should I do with my dose when I get a positive test?”

Never stop or change thyroid medicine on your own. If your test comes back normal, that's one less thing to wonder about. If it doesn't, you've found it at the best possible time. For the rest of the 90-day plan, see how to prepare your body for pregnancy.

Go deeper

Grow a Genius

In the book: Chapter 6: The Preconception Check-Up

Grow a Genius explains the thyroid, iron and blood sugar checks in plain language and gives you a printable take-along checklist with the exact questions to hand your doctor.

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