Hashimoto's and Trying to Conceive: why TSH should be below 2.5

Maybe you recognize yourself in this picture. Maybe you are the woman holding the lab report. Or the person beside her who would like to do something and does not know what.

In this article, I do not want to promise you anything that medicine cannot deliver. When a pregnancy does not happen, there are almost always several levers involved, and in both partners. The thyroid is one of them. But it is one that can be measured well and monitored well.

And that is exactly what this is about. A TSH of 3.5 can be perfectly fine in a woman without a thyroid condition. For a woman with Hashimoto's who already takes levothyroxine and wants to become pregnant, the guidelines name a different target: a TSH within the reference range, but below 2.5 mU/l.

Why? Not because 2.5 is some magic fertility threshold. But because pregnancy raises the need for thyroid hormone within the first few weeks. A healthy thyroid absorbs this on its own. A thyroid with Hashimoto's often cannot. Then the tablet has to make the difference, and a tablet does not adjust itself.

In this article, let's take a closer look together: What does the target stand for, who does it apply to, and who is it explicitly not meant for? What do antibodies mean for the risk of miscarriage? And what did three large trials, which had hoped for a simple solution, actually show? At the end, you will get a roadmap so that the crucial first weeks are not left to chance.

Why a TSH of 3.5 with Hashimoto's is a value without reserve

“Normal range” sounds like an all-clear. Like: You can stop worrying. For many people, that is true. But a normal range answers only one single question: Does this value lie where most healthy, non-pregnant people lie?

It does not answer the question that is on your mind right now. Namely: Is this value enough for what my body will soon have to do?

The current 2026 guideline of the American Thyroid Association (ATA) puts it very soberly. Reference ranges describe the normal spread within a population. They do not reflect the risk of complications.

TSH is a messenger, not a measuring cup

The TSH value does not measure your thyroid hormone. It shows how loudly your pituitary gland is calling for hormone. If enough arrives, it calls quietly, and TSH is low. If too little arrives, it calls louder, and TSH rises.

So a TSH of 3.5 on levothyroxine means: Your pituitary gland is already calling louder than would be necessary within the target range. For everyday life, that is often enough. For a pregnancy, there is little room left at the top.

What pregnancy demands of the thyroid

Imagine your thyroid as a small power plant. In everyday life, it delivers a steady supply of electricity. When a pregnancy begins, the city suddenly reports a much higher demand. And not for a few days, but for months.

The ATA guideline describes quite precisely where this extra demand comes from. Estrogen rises and drives up a transport protein called TBG, from about week 7 to a peak around week 16. TBG soaks up T4 like a sponge. To keep enough hormone freely available, more has to be produced.

At the same time, more T4 passes from you to the baby. The placenta also breaks down T4 with its own enzyme, type 3 deiodinase. And your kidneys excrete more iodine. This is the hormonal lens on this topic: several systems are pulling on the same resource at the same time.

A healthy thyroid gets a tailwind here. The pregnancy hormone hCG can dock onto the TSH receptor and boost production, most strongly toward the end of the first trimester. This way, the power plant ramps up on its own.

With Hashimoto's, some of the turbines are shut down. In women with TPO antibodies, the guideline describes a weaker thyroid response to hCG in early pregnancy. If you take levothyroxine, even more depends on the tablet. And a tablet does not increase itself.

The guideline's figures show how big the difference is. Most women who take levothyroxine for hypothyroidism need around 25 percent more by about week 12. By about week 20, it is around 50 percent more.

A worked example that makes the buffer visible

This becomes especially clear during fertility treatment. During hormonal stimulation of the ovaries, estrogen rises sharply. According to the ATA guideline, this raised TSH by an average of 1.5 mU/l in women on levothyroxine, and this rise persisted into pregnancy.

Now do the math with me. If you start at 2.0, you end up at about 3.5 and are still within limits. If you start at 3.5, you end up at about 5.0. That is above the upper limit the guideline gives for early pregnancy when the lab has no pregnancy-specific reference values of its own.

Why the first weeks matter so much for the baby's brain

Now comes the lens that moves me most on this topic: your baby's nervous system.

In the first half of pregnancy, the baby depends on your T4. Its own thyroid only starts production later. The baby's brain takes up T4 and converts it locally into active T3 itself. A bit like a baker who is not delivered finished bread, but flour, and processes it in their own bakery.

An older but still frequently cited study shows how seriously medicine takes this.

And now the news that often gets lost in this context. According to the ATA guideline, women with known hypothyroidism who are well controlled on levothyroxine have, based on current data, similar risks for pregnancy and baby as women without hypothyroidism. So being well controlled is not a luxury. It is your protection.

That is also why the guideline recommends only levothyroxine, meaning pure T4, in pregnancy. Preparations containing T3 or thyroid extract should not be used even when you are trying to conceive. You can read why further down in the roadmap.

What does this mean for you in practice? If you have Hashimoto's, take levothyroxine and want to become pregnant, do not just ask: Is my value normal? Ask: Which target applies to me, and am I within it right now?

And now you know why a 3.5 on your lab report can mean something different from the same number for your friend without a thyroid condition.

Three starting points: who the 2.5 applies to and who it does not

If you search the internet at night, you will quickly find sentences like: TSH must be below 2 when you are trying to conceive. Or below 1. Or: Antibodies mean tablets right away. That sounds decisive. But it confuses more than it clarifies. Because it lumps three very different situations together.

So before we talk about targets, a simple question is worth asking: Where do you stand right now?

By the way, the scene from the beginning belongs to starting point A. She takes levothyroxine, and her TSH is 3.5. This is exactly where the target range applies, and exactly where a conversation about the dose is worthwhile.

Where the 2.5 originally comes from

The number 2.5 has a history. For a long time, it was considered the upper limit for TSH in the first trimester of pregnancy. A study from southern Italy strongly fueled this idea in 2010.

A large analysis of fertility treatments from New York came to a similar result. Whether the TSH cutoff was drawn at 2.5 or at 4.5 made no difference to the rates of pregnancy, birth or miscarriage. With the stricter cutoff, however, almost five times as many women would have been classified as having thyroid disease.

In 2024, the American Society for Reproductive Medicine drew a clear conclusion from this. In its assessment, TSH values between 2.5 and 4 are not associated with an increased risk of miscarriage.

And what about “below 2”?

Maybe you have read that TSH should ideally be below 2 or even below 1 when you are trying to conceive. Some practices and fertility centers do in fact aim for the lower part of the range. However, I do not know of any study showing that a TSH of 1.8 on levothyroxine brings better chances than a TSH of 2.3.

The ATA guideline deliberately also sets a lower limit of 0.5. It points out that a standard dose increase after a positive test leads to overtreatment somewhat more often in women with a TSH below 1.5 before pregnancy. And in the follow-up of a large British-Italian study, children of women with signs of overtreatment had more symptoms of ADHD and behavioral problems, with similar cognitive development.

So lower is not automatically better. A range is a range.

And now you know why the same number can mean three different things in three situations.

Antibodies and miscarriage: a real risk, but not a hormone deficiency

Maybe you first learned about your TPO antibodies after a miscarriage. And then you read sentences like: Antibodies triple the risk. That hits hard. It can feel as if your own body has been working against you.

Let me say something first that matters to me. A miscarriage is not your fault. And elevated antibodies are not a verdict on your body. They are a measurement that makes a tendency of your immune system visible.

Tripling sounds dramatic. That is why a second number is so important: the absolute difference. The ATA guideline puts it at about 2 to 8 percentage points more miscarriages for women with antibodies compared with women without. That is real and deserves attention. But it is a difference in percentage points, not a destiny.

The same 2011 meta-analysis contained something else that raised hopes. Two small randomized trials suggested that levothyroxine might roughly halve miscarriages. The idea seemed obvious: If antibodies increase the risk, just add a little hormone. Then came the large trials.

Three large trials, one clear answer

The 2026 ATA guideline has drawn a clear conclusion from this. In women with antibodies and normal thyroid function, levothyroxine should not be offered: not when trying to conceive, not after miscarriages and not during pregnancy. The recommendation is rated as strong, and the quality of evidence as high.

I find one detail particularly revealing. The guideline found no group in the studies that did benefit after all. Neither a TSH above 2.5 nor previous miscarriages, age or antibody levels changed the result.

The previous 2017 version had still left open whether levothyroxine could be considered in women with antibodies and previous miscarriages. With the new data, that door is closed. This is not a step backward. It is medicine that keeps learning.

If not a hormone deficiency, then what?

From a KPNI perspective, this is where it gets interesting. If levothyroxine does not lower the risk, then the cause is apparently not a lack of thyroid hormone. The ATA guideline considers it plausible that thyroid antibodies reflect a more general tendency of the immune system toward autoimmunity.

Imagine the antibodies as a barometer. It shows that the weather in the immune system is changing. If you turn the needle, the weather does not change. Levothyroxine changes the hormone level. It does not reach the immune situation behind it.

Pregnancy already demands a balancing act from the immune system. It has to tolerate a baby who carries half foreign characteristics. The guideline describes that antibody levels therefore usually fall over the course of pregnancy. After birth, the immune system rebounds. That is exactly when women with antibodies have a higher risk of postpartum thyroiditis.

What I want to tell you honestly: According to the guideline, there are so far no data showing a better pregnancy outcome with antibodies for corticosteroids, immunoglobulins, a particular diet or selenium. Corticosteroids and immunoglobulins should therefore not be offered. If there are signs of other autoimmune diseases, however, the guideline considers it sensible to take a closer look even before pregnancy.

As a physician with a KPNI perspective, I still look at the whole immune system: sleep, stress load, signs of inflammation, other autoimmune diseases. Not because this has been shown to prevent miscarriages. Studies on that are lacking. But because these factors matter for your own health anyway, and a pregnancy might benefit from a calmer immune system. That is my clinical perspective, not a proven therapy. You can find more on this in the articles on autoimmune diseases and on what measurably changes Hashimoto's antibodies.

And now you know why antibodies should be taken seriously without a tablet automatically following.

The roadmap: close monitoring instead of “get in touch if anything comes up”

Imagine the moment: two lines on the test. Joy, disbelief, maybe a little fear too. The first appointment with your gynecologist is often still a few weeks away. And it is exactly during this time that the extra demand for thyroid hormone begins.

If nobody thinks about the thyroid then, valuable weeks pass. Not out of carelessness. But because there was no plan.

A large American trial shows why timing is so important.

The 2026 ATA guideline describes a clear sequence for women with treated hypothyroidism. I have translated it into seven stages for you. It does not replace a discussion with your doctor. But it gives you the right questions to ask.

One more important detail on the fourth stage. The guideline points out that a standard increase can be too much for some women: with a TSH below 1.5 before pregnancy, with a starting dose above 100 micrograms a day or when the weekly dose goes up by two whole tablets. For women who are treated only for mild subclinical hypothyroidism, there are no data on the standard increase. There, the dose is more likely adjusted according to measured values. That is exactly why the number belongs in a conversation with your doctor and not in a blog article.

One practical point often gets overlooked. Many supplements for conception and pregnancy contain iron or calcium. Both can interfere with the absorption of levothyroxine if you take them too close to the tablet. How to plan the gap is explained in the article Taking levothyroxine correctly.

And now you know why the plan for the positive test should be made before the test.

Iodine, selenium and iron: the building blocks around trying to conceive

By the time you are trying to conceive, if not before, you find yourself standing in front of a shelf full of supplements. Folic acid, iodine, selenium, iron, vitamin D, all in friendly packaging. And with Hashimoto's, there is a particular uncertainty on top: Am I even allowed to take iodine?

Iodine: relevant with Hashimoto's too

Iodine is the raw material your thyroid uses to make hormone. Later, your baby's thyroid needs iodine too. In pregnancy, your kidneys also excrete more iodine. This is the metabolic lens: demand rises while more is lost.

The 2026 ATA guideline advises all women who are trying to conceive, pregnant or breastfeeding to take a supplement of 150 micrograms of iodine a day, ideally starting at least three months before conception. Women who already get a lot of iodine through their diet are the exception. At the same time, a persistently very high iodine intake and supplements with more than 500 micrograms a day should be avoided in pregnancy.

According to the guideline, a deficiency is more likely if you do not use iodized table salt, eat hardly any dairy products or follow a vegan diet. As early as 2004, the Madrid research group led by Morreale de Escobar emphasized that mild iodine deficiency is a common cause of too little maternal T4 in Western countries.

Many women with Hashimoto's have heard that they should avoid iodine. However, the guideline's recommendation is aimed at all women in this phase of life. How this applies to you is best discussed with your doctor. You can find the background to the debate in the article Iodine with Hashimoto's and in Iodine and the thyroid.

Selenium: a trace element with two faces

The 2026 ATA guideline nevertheless takes a cautious view of the overall picture. For pregnant women with antibodies and normal thyroid function, it recommends not offering selenium, with moderate quality of evidence. Selenium has a narrow window between too little and too much. If at all, it belongs in a medical decision after your selenium status has been measured, not in your shopping basket just in case. More on this in the article Selenium and the thyroid.

Iron: the silent partner

The enzyme that builds iodine into thyroid hormone is called thyroid peroxidase. It is exactly the enzyme that TPO antibodies are directed against. And it is an iron-containing enzyme. Without iron, the workshop is missing its tools, so to speak.

In pregnancy, iron requirements rise considerably anyway. Whether and how iron is supplemented depends on your values. You can read what matters here in the article Iron deficiency in pregnancy. And remember to leave a gap between iron and your levothyroxine tablet.

And now you know why, with the building blocks, it is not the amount that counts, but the fit.

Trying to conceive is teamwork, and more than a lab value

Trying for a baby without success runs in cycles. Hoping, waiting, testing, being disappointed, hoping again. Many couples describe how at some point everything revolves around this one calendar. And how the thyroid appears in it like yet another test that has to be passed.

So let's put the thyroid fairly into context. Untreated marked hypothyroidism can disrupt the cycle. According to the ATA guideline, about 23 percent of women with untreated overt hypothyroidism have cycle disorders. And hypothyroidism is behind 2 to 3 percent of all cases in which ovulation does not occur.

With mildly elevated TSH and no treatment, meta-analyses of partly weaker studies describe a slightly lower chance of conception, in absolute terms about 1.4 to 4.5 percentage points. The risk of miscarriage was 0.4 to 0.7 percentage points higher. This shows two things: The thyroid is a real factor. And it is rarely the only one. You can find more about this connection in the article The thyroid and your cycle.

And then there is the second person in this calendar. Trying to conceive always involves two people. Sperm quality is just as much part of the equation, and there are factors there that can be investigated too. What matters is covered in the article Improving sperm quality.

Chronic stress also plays a part in the hormonal system, via the axis between the brain and the adrenal glands. The article Cortisol, stress and female hormones describes how this can affect the female cycle.

What it comes down to in the end

And for me, this is the heart of this article. It is not about a number on a lab report. It is about how the two of you experience the months of waiting.

Without a plan, every cycle feels like an exam whose rules nobody has explained. With a plan, you know which levers are in your hands and which are not. That does not take away every uncertainty. But it takes away the helplessness.

In anthroposophic medicine, there is the idea of thinking, feeling and willing. Applied to this path, it means: understanding what your lab values mean. Allowing yourselves to feel what this path does to you. And acting where action is possible.

Your three levers for the coming weeks

Do you remember the couple in the car? The question was: If everything is normal, why doesn't it feel that way? Perhaps the better question is: Does my value fit what I am planning? You are allowed to ask this question. And it deserves an answer.

And now you know why a TSH below 2.5 for women with Hashimoto's on levothyroxine is not a strict dogma, but a buffer meant to create room for the crucial weeks.

Frequently asked questions about Hashimoto's, trying to conceive and TSH

Further reading: related guides