Dr. Prathaneni Srividya

How Much Does an Underactive Thyroid Really Matter in Fertility?

When a couple has difficulty conceiving, <span>thyroid</span> checks are among the first things doctors do. Yet, it is an important fact that one often forgets - overt hypothyroidism or mild/subclinical hypothyroidism are actually very different things.

Having a thyroid that is not active enough means the body is not producing enough thyroid hormones. These hormones regulate metabolism and at the same time can greatly affect the functioning of the reproductive system like the occurrence and regularity of the <span>menstrual cycle</span>, ovulation and the hormonal environment that is necessary for pregnancy. Still, new findings suggest that we should not always take the blame for infertility just because the <span>TSH</span> level is borderline or abnormal.

<h2>When hypothyroidism genuinely affects <span rel="noindex">fertility</span></h2>

Undiagnosed, overt hypothyroidism has the potential to disrupt reproduction in many ways. Lower levels of thyroid hormones may alter the balance in hypothalamic-pituitary-ovarian axis, which would affect ovulation and menstruation. In severe cases of hypothyroidism, higher <span>prolactin</span> levels could be observed, disrupting endocrine signaling.

According to new 2026 American Thyroid Association guidelines, overt hypothyroidism is considered to be a known risk factor for infertility and recurrent miscarriages. Specifically, according to the guideline, menstrual problems have been seen in 23% of women with untreated overt hypothyroidism, and about 2–3% of all cases of anovulation are due to hypothyroidism. 

It is important to note that women with known <a href="https://www.healthandme.com/health-wellness/greys-anatomy-star-opens-up-about-hashimotos-hypothyroidism-know-how-this-autoimmune-disorder-can-impact-body-and-mind-article-152433582" data-type="tilCustomLink">hypothyroidism</a>, who received appropriate treatment with levothyroxine and were in a euthyroid state, had the same chances for fertility as women without thyroid disorder.

Thus, the message here is not that women with hypothyroidism are unable to conceive. It is that thyroid dysfunction should not go untreated during planning of pregnancy.

Also read: <a href="https://www.healthandme.com/nutrition/world-iodine-deficiency-day-significance-and-importance-article-151048437" data-type="tilCustomLink">World Iodine Deficiency Day: Significance And Importance</a>

<h2>But what about a mildly raised TSH?</h2>

This is where the science becomes considerably more interesting.

Subclinical hypothyroidism generally means that TSH is elevated while free T4 remains within the normal range. It is increasingly detected during fertility investigations, partly because women are being tested more frequently.

<h2>But does every mildly elevated TSH require treatment?</h2>

A 2025 review article published in Current Opinion in Obstetrics &amp; Gynecology concluded there isn't enough reliable evidence at this time to consider subclinical hypothyroidism as a contributor to infertility. It was also found that the use of levothyroxine was not consistently tied to improved <a href="https://www.healthandme.com/parenting/maternal-immunisation-the-forgotten-half-of-protecting-a-babys-health-article-156394188" data-type="tilCustomLink">pregnancy </a> or live-birth outcomes in women suffering from subclinical hypothyroidism. The authors advocated for screening and treatment practices that are selective rather than universal.

This new recommendation represents a clear departure from the previous approach that was often overly aggressive in treating women with marginal thyroid test results.

The same findings were reported in a randomized trial-based 2024 systematic review and meta-analysis, which was published in a leading international medical journal by the world's largest academic medical publisher. The analysis showed that preconception levothyroxine administration had no statistically significant effect on live birth rates or pregnancy or miscarriage rates in subclinical hypothyroid women. Still, it should be mentioned that the scientists have recognized this issue and they have pointed out that for women with a TSH level above 4 mIU/L, the evidence is even more uncertain and that better trials are needed.

Also read: <a href="https://www.healthandme.com/health-wellness/world-menopause-day-is-it-empty-nest-syndrome-or-menopause-symptoms-expert-explains-article-156361585" data-type="tilCustomLink">World Menopause Day: Is It Empty Nest Syndrome Or Menopause Symptoms? Expert Explains</a>

<h2>New 2026 <span rel="noindex">IVF</span> data add another layer</h2>

A quite relevant study published in <em>Human Reproduction</em> in July of the same year examined 412 women who were subclinically hypothyroid and had frozen embryo transfers. In this group, 198 got levothyroxine whereas the other 214 were untreated. The live-birth rates were 43.9% for levothyroxine users against 39.3% in the untreated group. Even though the numbers were better for the treated group, the difference was not statistically significant.

It is also remarkable that apart from the levothyroxine group, they got other embryological results as well, including more fertilised oocytes, Day-3 embryos, good-quality Day-3 embryos and blastocysts. Yet it didn't lead to statistical significance of the live births. It's a typical point in fertility medicine that even if lab or embryological indicators are enhanced, it doesn't necessarily mean having more children.

In addition, the study carried out in 2026 on women undergoing the procedure of assisted reproduction, who left untreated, revealed that women with untreated TSH between around 3.6 and 6 mIU/L retrieved fewer oocytes. Yet, the live-birth rates after accounting for other factors turned out to be no significant difference at all (1,2). 

In fact, there was quite a relevant study carried out recently on the effect of levothyroxine on the frozen embryo transfer (<span>FET</span>) outcomes in cases of subclinical hypothyroidism. It was conducted in <em>Human Reproduction</em> in July 2026. The participants of the study were 412 hypothyroid women who were undergoing FET (frozen embryo transfer).

<h2>Does thyroid disease affect egg quality?</h2>

Some evidence is emerging from here, but it should not yet be presented as an undisputed scientific fact.

On a big research scale with 2,867 women who were in the first IVF treatment cycle, it turned out that women who had subclinical or even overt hypothyroidism had their <a href="https://www.healthandme.com/health-wellness/could-ovarian-ageing-be-reversed-new-study-on-mice-shows-promise-article-156431862" data-type="tilCustomLink">ovarian function</a> decline much more than normal women, which was assessed through AMH and antral follicle count independent of thyroid autoimmunity. Though being an observational study, it just indicates the presence of a connection but not that hypothyroidism is causing the ovarian reserve to diminish.

The reason for drawing up such a clear line between them is that fertility depends on age, female factors like ovarian reserve, endometriosis PCOS, tubal factors, male factors like sperm quality, metabolic health and many other variables. Thyroid function is just one aspect out of a large puzzle not the complete picture

What about Hashimoto's and thyroid antibodies?

Thyroid autoimmunity is another contributing factor. There are cases where some females have thyroid antibodies, specifically thyroid peroxidase antibodies, even when the thyroid hormones of the individual are within normal range.

There is an inconclusive body of evidence associating the existence of thyroid antibodies with difficulty conceiving. In a review conducted in 2025, there was no substantial evidence that would support routine screening for every infertile female. The more recent 2026 ATA guidelines highlight the importance of taking into consideration the individual needs of the patient.

<h2>So, should every woman trying to conceive have a thyroid test?</h2>

A thyroid assessment is particularly important for women with a previous thyroid disorder, symptoms suggestive of hypothyroidism, irregular menstrual cycles, infertility, recurrent pregnancy loss, autoimmune disease or a family history of thyroid disease.

Recent Indian health coverage has also highlighted the importance of checking thyroid function during preconception care. A January 2026 Hindustan Times report noted the association between thyroid dysfunction and menstrual irregularities, infertility and recurrent miscarriage, while recommending greater awareness around testing during preconception and early pregnancy.

Male Infertility, IVF Myths & Biological Clock | IVF Pioneer Answers

Thyroid gland is the most misused organ when it comes to health, Mainly when it comes to treatment.

When a woman suffers from uncorrected overt hypothyroidism, this can clearly interfere with fertilization by causing ovulation and monthly cycle irregularities and affecting fertility generally, which needs to be corrected before pregnancy whenever it is feasible. Generally, fertility results are encouraging after <a href="https://www.healthandme.com/health-wellness/why-two-people-with-the-same-thyroid-cancer-can-need-completely-different-treatments-article-156306605" data-type="tilCustomLink">thyroid</a> hormone levels are kept within the normal range.

In case of very mild to subclinical hypothyroidism In particular when the TSH is only a little bit above the normal levels and free T4 remains stable, the situation is far from a textbook case and it is rather a grey zone. However, new research conducted in 2025 and 2026 increasingly supports the idea of going beyond a one-size-fits-all prescription of levothyroxine to women with borderline TSH.

The only reasonable course of action for a woman who wants to conceive is as simple as: assess thyroid function clinically only when it is necessary (i.e., when there are clinical signs/symptoms of thyorid imbalance/ disease). Determine if the imbalance is one of those that are clinically overt (like signs and symptoms of overt hypothyroidism, elevated TSH, etc.) or clinically subclinical, then consider the TSH level, symptoms, thyroid antibodies, reproductive history, etc., and refer the treatment to an endocrinologist or fertility specialist to decide.

The goal is not to chase an arbitrary “perfect” TSH number. The goal is to identify genuine thyroid disease, treat it appropriately and avoid allowing either an untreated disorder or unnecessary treatment to become another source of anxiety during an already stressful fertility journey.

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Perimenopause Is Not Just About Periods: The Metabolic Changes Women Should Know About

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When a couple has difficulty conceiving, thyroid checks are among the first things doctors do. Yet, it is an important fact that one often forgets - overt hypothyroidism or mild/subclinical hypothyroidism are actually very different things.Having a thyroid that is not active enough means the body is not producing enough thyroid hormones. These hormones regulate metabolism and at the same time can greatly affect the functioning of the reproductive system like the occurrence and regularity of the menstrual cycle, ovulation and the hormonal environment that is necessary for pregnancy. Still, new findings suggest that we should not always take the blame for infertility just because the TSH level is borderline or abnormal.When hypothyroidism genuinely affects fertilityUndiagnosed, overt hypothyroidism has the potential to disrupt reproduction in many ways. Lower levels of thyroid hormones may alter the balance in hypothalamic-pituitary-ovarian axis, which would affect ovulation and menstruation. In severe cases of hypothyroidism, higher prolactin levels could be observed, disrupting endocrine signaling.According to new 2026 American Thyroid Association guidelines, overt hypothyroidism is considered to be a known risk factor for infertility and recurrent miscarriages. Specifically, according to the guideline, menstrual problems have been seen in 23% of women with untreated overt hypothyroidism, and about 2–3% of all cases of anovulation are due to hypothyroidism. It is important to note that women with known hypothyroidism, who received appropriate treatment with levothyroxine and were in a euthyroid state, had the same chances for fertility as women without thyroid disorder.Thus, the message here is not that women with hypothyroidism are unable to conceive. It is that thyroid dysfunction should not go untreated during planning of pregnancy.Also read: World Iodine Deficiency Day: Significance And ImportanceBut what about a mildly raised TSH?This is where the science becomes considerably more interesting.Subclinical hypothyroidism generally means that TSH is elevated while free T4 remains within the normal range. It is increasingly detected during fertility investigations, partly because women are being tested more frequently.But does every mildly elevated TSH require treatment?A 2025 review article published in Current Opinion in Obstetrics & Gynecology concluded there isn't enough reliable evidence at this time to consider subclinical hypothyroidism as a contributor to infertility. It was also found that the use of levothyroxine was not consistently tied to improved pregnancy  or live-birth outcomes in women suffering from subclinical hypothyroidism. The authors advocated for screening and treatment practices that are selective rather than universal.This new recommendation represents a clear departure from the previous approach that was often overly aggressive in treating women with marginal thyroid test results.The same findings were reported in a randomized trial-based 2024 systematic review and meta-analysis, which was published in a leading international medical journal by the world's largest academic medical publisher. The analysis showed that preconception levothyroxine administration had no statistically significant effect on live birth rates or pregnancy or miscarriage rates in subclinical hypothyroid women. Still, it should be mentioned that the scientists have recognized this issue and they have pointed out that for women with a TSH level above 4 mIU/L, the evidence is even more uncertain and that better trials are needed.Also read: World Menopause Day: Is It Empty Nest Syndrome Or Menopause Symptoms? Expert ExplainsNew 2026 IVF data add another layerA quite relevant study published in Human Reproduction in July of the same year examined 412 women who were subclinically hypothyroid and had frozen embryo transfers. In this group, 198 got levothyroxine whereas the other 214 were untreated. The live-birth rates were 43.9% for levothyroxine users against 39.3% in the untreated group. Even though the numbers were better for the treated group, the difference was not statistically significant.It is also remarkable that apart from the levothyroxine group, they got other embryological results as well, including more fertilised oocytes, Day-3 embryos, good-quality Day-3 embryos and blastocysts. Yet it didn't lead to statistical significance of the live births. It's a typical point in fertility medicine that even if lab or embryological indicators are enhanced, it doesn't necessarily mean having more children.In addition, the study carried out in 2026 on women undergoing the procedure of assisted reproduction, who left untreated, revealed that women with untreated TSH between around 3.6 and 6 mIU/L retrieved fewer oocytes. Yet, the live-birth rates after accounting for other factors turned out to be no significant difference at all (1,2). In fact, there was quite a relevant study carried out recently on the effect of levothyroxine on the frozen embryo transfer (FET) outcomes in cases of subclinical hypothyroidism. It was conducted in Human Reproduction in July 2026. The participants of the study were 412 hypothyroid women who were undergoing FET (frozen embryo transfer).Does thyroid disease affect egg quality?Some evidence is emerging from here, but it should not yet be presented as an undisputed scientific fact.On a big research scale with 2,867 women who were in the first IVF treatment cycle, it turned out that women who had subclinical or even overt hypothyroidism had their ovarian function decline much more than normal women, which was assessed through AMH and antral follicle count independent of thyroid autoimmunity. Though being an observational study, it just indicates the presence of a connection but not that hypothyroidism is causing the ovarian reserve to diminish.The reason for drawing up such a clear line between them is that fertility depends on age, female factors like ovarian reserve, endometriosis PCOS, tubal factors, male factors like sperm quality, metabolic health and many other variables. Thyroid function is just one aspect out of a large puzzle not the complete pictureWhat about Hashimoto's and thyroid antibodies?Thyroid autoimmunity is another contributing factor. There are cases where some females have thyroid antibodies, specifically thyroid peroxidase antibodies, even when the thyroid hormones of the individual are within normal range.There is an inconclusive body of evidence associating the existence of thyroid antibodies with difficulty conceiving. In a review conducted in 2025, there was no substantial evidence that would support routine screening for every infertile female. The more recent 2026 ATA guidelines highlight the importance of taking into consideration the individual needs of the patient.So, should every woman trying to conceive have a thyroid test?Not necessarily in exactly the same way.A thyroid assessment is particularly important for women with a previous thyroid disorder, symptoms suggestive of hypothyroidism, irregular menstrual cycles, infertility, recurrent pregnancy loss, autoimmune disease or a family history of thyroid disease.Recent Indian health coverage has also highlighted the importance of checking thyroid function during preconception care. A January 2026 Hindustan Times report noted the association between thyroid dysfunction and menstrual irregularities, infertility and recurrent miscarriage, while recommending greater awareness around testing during preconception and early pregnancy.The bottom line         Thyroid gland is the most misused organ when it comes to health, Mainly when it comes to treatment.When a woman suffers from uncorrected overt hypothyroidism, this can clearly interfere with fertilization by causing ovulation and monthly cycle irregularities and affecting fertility generally, which needs to be corrected before pregnancy whenever it is feasible. Generally, fertility results are encouraging after thyroid hormone levels are kept within the normal range.In case of very mild to subclinical hypothyroidism In particular when the TSH is only a little bit above the normal levels and free T4 remains stable, the situation is far from a textbook case and it is rather a grey zone. However, new research conducted in 2025 and 2026 increasingly supports the idea of going beyond a one-size-fits-all prescription of levothyroxine to women with borderline TSH.The only reasonable course of action for a woman who wants to conceive is as simple as: assess thyroid function clinically only when it is necessary (i.e., when there are clinical signs/symptoms of thyorid imbalance/ disease). Determine if the imbalance is one of those that are clinically overt (like signs and symptoms of overt hypothyroidism, elevated TSH, etc.) or clinically subclinical, then consider the TSH level, symptoms, thyroid antibodies, reproductive history, etc., and refer the treatment to an endocrinologist or fertility specialist to decide.The goal is not to chase an arbitrary “perfect” TSH number. The goal is to identify genuine thyroid disease, treat it appropriately and avoid allowing either an untreated disorder or unnecessary treatment to become another source of anxiety during an already stressful fertility journey.

how much does an underactive thyroid really matter in fertility?

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