Hashimoto's and Infertility: The Connection Few Investigate

The couple had been trying to conceive for eighteen months when they came to my office in Campinas, after a full battery of gynecological tests and one unsuccessful attempt at assisted reproduction. Their reproductive hormone tests had come back normal, her ovarian reserve was as expected for her age, and the semen analysis showed no relevant abnormalities either. What no one had ordered until then was an anti-TPO test.

This kind of gap in the workup is more common than you might think. Fertility evaluations usually follow a well-established protocol — reproductive hormones, anatomical assessment, semen analysis — but the thyroid, especially its autoimmune form, is often left out of that first round of testing, even though there is consistent scientific evidence linking the two conditions.

Why the thyroid affects fertility

Thyroid hormones are directly involved in regulating the menstrual cycle, ovulation, and keeping the uterine lining in the right condition for an embryo to implant. Changes in thyroid function — even subtle ones — can translate into irregular cycles, ovulation failure, and a less favorable uterine environment, which helps explain why thyroid dysfunction has historically been associated with reproductive difficulties.

What more recent research has added to this picture is that the autoimmune process of Hashimoto's thyroiditis seems to play a role of its own, separate from and in addition to the purely hormonal effect — and that changes how the workup should be done.

Thyroid antibodies can interfere even when hormone levels are normal

Observational studies show that women who test positive for thyroid antibodies (anti-TPO or anti-TG), even when they are euthyroid — meaning their TSH and free T4 are within normal limits — have higher rates of infertility, a greater risk of miscarriage, and, in assisted reproduction settings, lower success rates compared with women without these antibodies.

The explanation is still being actively researched, but the most widely accepted hypotheses include a higher level of systemic inflammation associated with autoimmunity, an increased risk that thyroid function will decompensate during pregnancy — a period of high hormonal demand — and possible direct interference by the antibodies in the uterine environment and embryo implantation.

Why this investigation often comes too late

Most infertility workup protocols rely on TSH as the initial screening test, and when that result is normal, the thyroid is usually ruled out as a relevant cause. The problem is that this screening, on its own, doesn't detect thyroid autoimmunity — which requires specific testing for anti-TPO and anti-TG antibodies, tests that aren't always ordered systematically in this context.

That means couples may go through entire cycles of trying — including assisted reproduction, with all the physical, emotional, and financial toll it involves — without this specific piece of the puzzle ever really being investigated.

A normal TSH rules out overt hypothyroidism, but it does not rule out thyroid autoimmunity — and that distinction can be decisive in a fertility workup.

How functional clinical reasoning approaches this overlap

When a couple is struggling to conceive with no apparent cause, it makes clinical sense to include thyroid antibodies and free T4 in the workup, in addition to TSH — looking not only for overt hormonal dysfunction but also for signs of autoimmunity that may be quietly interfering with the process. This reasoning adds to — it does not replace — the gynecological and reproductive workup already underway.

When thyroid autoimmunity is identified, joint follow-up between the reproductive specialist and a thyroid-focused physician is usually the most efficient path, especially during pregnancy, when the demand for thyroid hormone rises significantly and the risk of decompensation is higher.

Individualized care is the key

Not every difficulty getting pregnant has a thyroid origin, and a fertility workup involves multiple fronts that need to be addressed together. But ruling out the thyroid based on a single TSH, without checking thyroid antibodies, leaves a real gap in the investigation — a gap that, for many couples, makes all the difference in how long it takes and which path they follow to pregnancy.

In the next article in this series, we'll talk about a time when thyroid vigilance should increase, not decrease: the postpartum period, and why postpartum thyroiditis so often goes unnoticed.