Hashimoto's and trying to conceive – what does current research say?

Many patients come to us worried that their Hashimoto's thyroiditis makes pregnancy impossible or significantly increases the risk of miscarriage. This worry is understandable, but only partly justified. The research picture is more nuanced than many articles on the internet suggest. What do we really know? And what does it mean for your treatment?

What is Hashimoto's thyroiditis?

Hashimoto's thyroiditis is an autoimmune disease in which the immune system attacks the body's own thyroid gland. It is the most common thyroid disease in women of reproductive age. Typical features are raised thyroid antibodies – above all TPO antibodies (Anti-TPO) – and a thyroid structure that looks altered on ultrasound.

In the early stages, thyroid values (TSH, fT3, fT4) often remain within the normal range. This is then called euthyroid Hashimoto's thyroiditis – the thyroid is still working normally, but the immune system is already attacking it.

If the disease progresses, an underactive thyroid can develop, which is then treated with L-Thyroxin.

What effect do TPO antibodies have on pregnancy and fertility?

TPO antibodies are associated with an increased risk of miscarriage and preterm birth. This is shown by a meta-analysis by Thangaratinam et al. (BMJ 2011) covering more than 30 studies. The association also applies to women with normal thyroid function.

The cause has not been fully clarified. It is debated whether the antibodies act directly or whether they are an expression of a general disturbance of immune regulation that also affects the implantation of the embryo.

The decisive question: does L-Thyroxin help when thyroid function is normal?

This is the question that concerns patients and doctors most. And here the evidence is clear:

TABLET trial (Dhillon-Smith et al., NEJM 2019)

In this randomised, double-blind, placebo-controlled trial, 952 euthyroid women with positive TPO antibodies who were trying to conceive or had a history of miscarriage were randomised (476 L-Thyroxin 50 µg/day, 476 placebo). The live birth rate was almost identical in both groups (37.4% vs. 37.9%, p=0.74) – no significant difference.

In a subgroup analysis, women with above-average TPO-AK levels (upper half of the study population) showed a signal of a reduced miscarriage rate under L-Thyroxin (RR 0.66, p=0.04). However, this effect was not significant for the live birth rate and could not be reproduced in the overall population. In the literature it is regarded as hypothesis-generating, not as a treatment recommendation.

T4LIFE trial (van Dijk et al., Lancet Diabetes & Endocrinology 2022)

In this multicentre, double-blind phase 3 trial in women with recurrent miscarriage and positive TPO antibodies, too, L-Thyroxin treatment showed no advantage over placebo with regard to the live birth rate.

What does this mean?

In euthyroid Hashimoto's thyroiditis, i.e. normal thyroid function despite positive antibodies, routine L-Thyroxin is not recommended on the current evidence. The trial data speak against it.

The situation is different if:

• The TSH value is raised (subclinical or overt hypothyroidism) – then L-Thyroxin is indicated
• The TSH value is in the upper normal range (> 2.5 mIU/L) with positive antibodies and a wish to conceive at the same time. Here an individual medical decision makes sense
• A pregnancy has begun. Thyroid function should then be monitored closely, because the requirement rises in early pregnancy

What TSH value should you aim for when trying to conceive?

With known Hashimoto's thyroiditis and a wish to conceive, current guidelines recommend adjusting the TSH value to the lower normal range, as a rule up to 2.5 mIU/L.

What about selenium?

Selenium is a trace element involved in the production of thyroid hormones. Some studies show that selenium supplementation can lower TPO antibody titres. However, a clinically relevant effect on pregnancy rates or miscarriages has not yet been demonstrated in high-quality randomised trials. Taking selenium in amounts of up to 200 µg daily is classed as safe.

What we would like to tell you

Hashimoto's is not a diagnosis that automatically worsens your chances of pregnancy. Many patients with Hashimoto's become pregnant without complications and carry their pregnancy to term in good health. What matters is that thyroid function is checked regularly and treated when necessary.

If you have positive TPO antibodies but normal thyroid values and are wondering whether you should take L-Thyroxin – our honest answer based on the current data is: not routinely. But an individual assessment of your situation, your values and your history always makes sense.

Dr. med. Wibke Wilkening is managing physician at Praxis für Fertilität – Kinderwunsch- und EndometrioseZentrum Berlin.

Sources

• Thangaratinam S et al. (2011): Association between thyroid autoantibodies and miscarriage and preterm birth. BMJ 342:d2616
• Dhillon-Smith RK et al. (2019): Levothyroxine in women with thyroid peroxidase antibodies before conception (TABLET). N Engl J Med 380:1316–1325
• van Dijk MM et al. (2022): Levothyroxine in euthyroid TPO-Ab positive women with recurrent pregnancy loss (T4LIFE). Lancet Diabetes Endocrinol 10:322–329
• Man R, Dhillon-Smith RK, Korevaar TIM (2025): Levothyroxine supplementation trials in preconception and pregnant women. Eur J Endocrinol 192:R7–R15
• AWMF S2k-Leitlinie (2025): Diagnostik und Therapie von Schilddrüsenerkrankungen bei Kinderwunsch und Schwangerschaft