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  • Writer's pictureDr. H. Singh, ND

A Potential Hidden Cause of Unexplained Infertility: Subclinical Hypothyroidism

Updated: Nov 22, 2021

Thyroid disease is estimated to be the second most common endocrinological disease that affects Women of reproductive age. In fact, it might be more common among patients that experience infertility. There is research suggesting that patients with other reproductive conditions, like PCOS for example, are actually at a higher risk of developing auto-immune thyroid disease. It may also be present in patients that have been diagnosed with Unexplained Infertility.

Thyroid disorders can, in general, be divided in to two different categories: hyperthyroid and hypothyroid disease. Hyperthyroid disease is when the thyroid gland starts to produce excess amounts of thyroid hormone while hypothyroid disease is when the thyroid gland produces insufficient amounts of thyroid hormone. However, we are seeing research now suggesting that patients may experience difficulty conceiving or may have an elevated risk for miscarriage with subclinical hypothyroidism and/or euthyroid auto-immune thyroid disease.

How is Subclinical Hypothyroidism diagnosed?

Subclinical hypothyroidism is when there are normal levels of the T4 thyroid hormone on a blood test accompanied with a mildly elevated thyroid stimulating hormone (TSH) test result. There may be mild symptoms of hypothyroidism present. In many cases, however, patients with subclinical hypothyroidism have no symptoms of hypothyroidism (fatigue, weight gain, cold intolerance, brain fog, or constipation to name a few of the possible symptoms).

The risk of having subclinical hypothyroidism is higher in patients that experience infertility and the risk also seems to increase with age.

What is Auto-immune Thyroid Disease?

There are two thyroid antibodies that, when elevated, may be associated with an increased risk for miscarriage. The anti-thyroid peroxidase antibody is the most commonly tested antibody and the anti-thyroglobulin antibody may also be tested. When either one of these antibodies are elevated, auto-immune thyroid disease is diagnosed. This may occur in the presence of normal amounts of thyroid hormone as well (euthyroid).

Patients with elevated thyroid antibodies are more likely to develop more severe hypothyroidism, and the higher the antibody levels the higher the risk. The risk of having a higher TSH level also increases with higher thyroid antibody levels.

What are normal TSH levels during pregnancy?

Current guidelines suggest that serum TSH levels should ideally be below 2.5 mIU/L during the first trimester and below 3.0 mIU/L during the second and third trimesters. Usually it is recommended for healthcare providers to increase the dose of thyroid medication for patients once they are pregnant as increasing pregnancy hormones can further slow thyroid function.

How does Thyroid Hormone impact Fertility?

Thyroid hormone receptors are located in various parts of the reproductive system. It has been observed that thyroid hormones may be necessary for the growth of follicles/eggs, support hormone (progesterone) production, and the maturation of trophoblastic cells (which are responsible for placental development).

Subclinical hypothyroidism has been associated with having up to 50% fewer fertilized eggs during in-vitro fertilization (IVF) or intracytoplasmic injection (ICSI) and an elevated TSH has been associated with lower success rates overall with IVF.

It has been suggested that supplementing with thyroid hormone in patients with subclinical hypothyroidism is associated with better embryo quality, increased implantation and live birth rates (number of patients that successfully conceive and go on to give a live birth).

It has also been suggested that supplementing with thyroid hormone in patients with elevated thyroid antibodies can help to reduce their risk of miscarriage.

Conflicting results in research around treating subclinical hypothyroidism and auto-immune thyroid disease:

Some research has shown that there is little benefit to treating auto-immune thyroid disease when thyroid levels are within normal range or treating subclinical hypothyroidism as it will not decrease miscarriage rates of improve live birth rates, however, the majority of the available research still suggests an improvement in both of these parameters.


  1. Myneni, R., Chawla, H. V., Grewal, A., Vivekanandan, G., Ndakotsu, A., Abubacker, A. P., Iqbal, A., & Khan, S. (2021). Thyroxine replacement for Subfertile females with subclinical hypothyroidism and autoimmune thyroiditis: A systematic review. Cureus.

This article is being shared as educational content and is in no way a replacement for medical advice or medical care, it is advised that anyone concerned about their Health should speak with their Naturopathic Doctor. Please discuss with your healtcare provider and only make changes to your medications regimen if recommended by your doctor and under their guidance.

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