Trying to Conceive with Thyroid Antibodies? What the TABLET Trial Found.
When you’re trying to conceive, a positive thyroid-antibody test can raise a lot of questions.
Will it make pregnancy harder? Could it increase the risk of miscarriage? Should you take thyroid medication - even if your thyroid hormone levels are normal?
The TABLET trial was designed to answer that last question. Its main finding was clear: giving levothyroxine to women with positive TPO antibodies and normal thyroid function did not improve their chances of having a live birth.
But that is not the whole story.
A later analysis found that some participants developed an underactive thyroid while trying to conceive or during pregnancy—and most of those cases appeared before pregnancy began. Together, these findings offer an important message: positive antibodies do not automatically mean you need medication, but they can be a reason to monitor thyroid function rather than waiting until you’re pregnant.[pubmed.ncbi.nlm.nih]
First, what are TPO antibodies?
Thyroid peroxidase antibodies, usually called TPO antibodies, are a marker of thyroid autoimmunity. You can have them even when your thyroid is still producing an appropriate amount of hormone. That normal-function state is called “euthyroid.”[btf-thyroid]
This distinction matters because an antibody test and a thyroid-function test answer different questions.
TPO antibodies help identify an autoimmune response involving the thyroid.
TSH and free T4 help show whether your thyroid is currently meeting your body’s hormone needs.[pubmed.ncbi.nlm.nih][academic.oup]
A positive antibody result is useful information. It is not, on its own, a diagnosis of an underactive thyroid or a prediction that you will struggle to conceive.
What was the TABLET trial?
TABLET stands for Thyroid AntiBodies and LEvoThyroxine. Published in 2019, it was a randomized, double-blind, placebo-controlled trial—meaning participants were randomly assigned to treatment, and neither they nor the clinicians assessing them knew who was receiving medication versus placebo.
Researchers enrolled 952 women who:
Had positive TPO antibodies.
Had normal thyroid function.
Had a history of miscarriage or infertility.
Were trying to conceive, either naturally or with fertility treatment.
Half were assigned 50 micrograms of levothyroxine daily, and half received placebo. Treatment began before conception and continued through pregnancy. Researchers wanted to know whether levothyroxine increased the likelihood of a live birth at or beyond 34 weeks.
This was not a trial of treating established hypothyroidism. It asked whether adding thyroid hormone would help women whose thyroid function was already normal.
Did thyroid medication improve pregnancy outcomes?
No significant benefit was found.
Among the 940 participants with primary-outcome data, the results were:
GroupLive births at or beyond 34 weeksLevothyroxine176 of 470 women: 37.4%Placebo178 of 470 women: 37.9%
The difference was not statistically significant. Researchers also found no significant differences in pregnancy loss or preterm birth.
These percentages were calculated across all participants in the analysis, including those who did not conceive - not just those who became pregnant. They should not be interpreted as an individual pregnant woman’s chance of a live birth.
For women with positive antibodies but normal thyroid function, this trial did not support routinely adding levothyroxine simply to improve pregnancy outcomes. It does not mean thyroid medication is unnecessary when hypothyroidism develops.
The finding that deserves more attention before pregnancy
A later analysis of TABLET participants, published in the Journal of Clinical Endocrinology & Metabolism, examined who developed hypothyroidism during follow-up.
Among 940 women whose thyroid function was initially normal:
70 women - 7.4% - developed hypothyroidism.
63 developed subclinical hypothyroidism.
Seven developed overt hypothyroidism.
Approximately 83% of these cases appeared before pregnancy.
Subclinical hypothyroidism means TSH is elevated while free T4 remains within the reference range. Overt hypothyroidism generally involves elevated TSH with low free T4.
The women who developed hypothyroidism had higher starting TSH levels and higher TPO-antibody levels than those who remained euthyroid.
That does not make antibody levels a crystal ball. It does show why an initially normal thyroid test may not be the end of the conversation.
The 7.4% figure also needs context: it came from this particular group of antibody-positive women with miscarriage or infertility histories, and included participants receiving levothyroxine as well as placebo. It is not a universal risk estimate for everyone trying to conceive (yet not all clinics are testing full thryoid panels).
What does this mean if you’re TTC?
The practical message is monitoring, not automatic medication.
If you know you have positive TPO antibodies, discuss a thyroid-function monitoring plan with your clinician while you’re trying to conceive. The later TABLET analysis specifically recommended regular monitoring during this period so hypothyroidism can be identified and treated promptly.
That does not mean repeatedly checking antibody levels every month. The key question is whether thyroid function has changed.
Once pregnancy is confirmed, American Thyroid Association guidance recommends TSH testing for antibody-positive women approximately every four weeks through midpregnancy, and at least once near 30 weeks. Your clinician may adjust the schedule according to your results and treatment needs.
Before conception, the appropriate testing interval should be individualized; the TABLET findings do not establish one universal schedule for every woman TTC.
Did developing hypothyroidism affect conception?
In the later analysis, women who developed hypothyroidism and received treatment were less likely to conceive than women whose thyroid function remained normal. However, the study did not find a significant difference in its live-birth outcome between these groups.
This finding needs careful interpretation. Developing hypothyroidism was not randomly assigned, so the analysis cannot prove that thyroid dysfunction alone caused the lower conception rate.
It does reinforce the importance of recognizing thyroid changes early. It does not show that preventive levothyroxine improves fertility in everyone with positive antibodies.
What the TABLET trial does not tell us
TABLET does not show that thyroid antibodies are harmless. It shows that one particular strategy - giving a fixed dose of levothyroxine to women with normal thyroid function - did not improve live-birth rates in the population studied.
It also did not test whether lowering antibodies through supplements, dietary changes, or photobiomodulation improves fertility or pregnancy outcomes. Those claims require their own evidence.
If you already take levothyroxine for diagnosed hypothyroidism, these findings are not a reason to stop it. The trial’s central question was about women whose thyroid function was normal before treatment, not whether necessary hormone replacement should be withheld.
Questions to bring to your next appointment
If you’re trying to conceive and have positive thyroid antibodies, consider asking:
“What do my TSH and free T4 results tell us about my current thyroid function?”
“How often should we repeat thyroid-function testing while I’m trying?”
“What should I do as soon as I have a positive pregnancy test?”
“Which changes in my results would make treatment appropriate?”
“What follow-up will I need after delivery?”
These questions help turn a concerning lab result into a clear plan.
A positive antibody test is information - not a verdict
Trying to conceive can already involve enough uncertainty. You do not need an antibody result presented as a guarantee of difficulty, or a promise that one treatment will solve everything.
The TABLET trial offers a more useful message: thyroid hormone treatment is not automatically beneficial when thyroid function is normal. At the same time, thyroid function can change before pregnancy, making appropriate follow-up worthwhile.
The goal is not to chase an antibody number. It is to recognize thyroid dysfunction when it occurs and provide the care you actually need.
Tracy
More resources on red light therapy for thyroid health.
https://blog.tracydonegan.org/blog/red-light-therapy-for-hashimotos-what-ultrasounds-reveal
https://blog.tracydonegan.org/blog/thyroid-testing-red-light-therapy-and-preparing-for-pregnancy
This article is for education and does not replace individualized preconception, fertility, or thyroid care. Do not start, stop, or adjust thyroid medication without your prescribing clinician.
