Your thyroid is a small gland with a huge impact on your body’s overall health. Located at the base of your neck, it produces hormones that affect and influence an extensive list of vital everyday functions, including metabolism, breathing, heart rate and cardiac function, muscle strength, the central and peripheral nervous systems, body temperature, weight management, cholesterol levels, and so many more (Thyroid Gland: Overview, 2019). It’s safe to say the thyroid impacts almost everything within our bodies; each of our cells contains nuclear “lock and key” receptors for thyroid hormones, meaning that every cell is capable of responding to the signal a thyroid hormone sends out when it wants to produce. The fact that thyroid hormones are seen reflected in these receptors and are present in every cell truly indicates just how important thyroid hormone levels are to our bodies.

But what possible role could such a small gland in the neck have on fertility? As it turns out; a big one. Thyroid dysfunction has been linked to a number of infertility concerns including issues with ovulation, implantation failure and pregnancy loss.

There are a variety of metrics that can be used to measure thyroid function and its impacts on fertility, and this article will explore the most common and important ones. However, in order to understand just how big these impacts are, we first need to look at how the thyroid works.

Your Thyroid: How It Works

The thyroid is part of the endocrine system, whose main purpose is to create, store, and release hormones out to the whole body through the bloodstream. Your brain sends a signal to the thyroid and tells it to start producing via a hormone called thyroid stimulating hormone (TSH). Upon receiving the TSH call, your thyroid takes iodine and uses it to create two key hormones: thyroxine (also known as T4, a storage hormone) and triiodothyronine (or T3, an active hormone). The 4 and 3 in these hormones’ names refer to the number of iodine attached to them. The main function of T4 is to become activated into T3. This occurs in a variety of different tissues like the liver and the gut, where an iodine is removed from the T4 and converts it into T3. This T3 then becomes active, sparking a series of changes that affects hormone levels and balances, energy levels, metabolic processes, and more.

Thyroid and the Immune System: What’s Fertility Got to Do with It?

The immune system is a key metric when assessing and understanding how thyroid function can impact fertility. The body’s natural way of assessing and managing its health, the immune system constantly circulates throughout the body, sampling and checking whether cells are healthy tissue or whether they’re an unwanted guest like a bacteria or a virus. When bacterial or viral tissue is found, the immune system perceives that our bodies are in danger and sends out antibodies — little protein warriors also known as immunoglobulins — to flag and neutralize the threat.

There are cases where the immune system gets confused and is unable to differentiate between foreign cells and its own, causing it to target healthy tissue. This is called an autoimmune attack. Research shows that approximately 1 in 8 women experience autoimmune issues where their body develops antibodies against their own thyroid. In these instances, whenever the immune system scans thyroid tissue, it erroneously perceives and flags it as “bad” and therefore a threat. This flag automatically sends the immune system into kill mode where it releases antibodies out to attack the thyroid, resulting in tissue damage and inflammation that can prevent the thyroid from functioning properly. Thyroid dysfunction is linked to low or insufficient levels of hormone production.  When it comes to fertility, proper hormone production and levels are essential in order for our bodies to be able to reproduce. As such, thyroid antibodies impact fertility in two significant ways:

• Setting the foundation for a pregnancy: damage to the thyroid can result in its inability to make enough hormones. If your body cannot produce sufficient levels of thyroid hormones, it won’t have the energy required to develop and ovulate a proper oocyte (egg).
• Keeping a pregnancy: thyroid dysfunction can result in not having sufficient energy to produce the amounts of estrogen and progesterone required to carry a pregnancy.

Fertility and Hashimoto’s Thyroiditis

Hashimoto’s thyroiditis is a common autoimmune disease where the immune system develops significantly high levels of thyroid antibodies. These antibodies attack and cause so much damage and inflammation to the thyroid that it is no longer capable of producing and keeping up with the amounts of hormones the body needs to function (Hashimoto’s Disease, 2020). Hashimoto’s disease is highly associated with recurrent pregnancy loss and recurrent implantation failure, not only due to the lack of sufficient hormone levels, but also due to the amount of inflammation caused by damaged thyroid tissue.

Inflammation and Unexplained Autoimmune Infertility

Inflammation is another key contributor to fertility due to its association with changing and impacting the lining of the uterus. The health of the uterus and uterine lining is crucial, and any changes can have a substantial impact on fertility. Inflammation caused by thyroid autoimmunity has been linked to autoimmune infertility and is often lumped into what is referred to as “unexplained infertility.” Changes in the lining of the uterus caused by an autoimmune response can contribute to the body not allowing proper implantation and/or proper embryo development.

Thyroid Antibodies and Premature Ovarian Insufficiency

When it comes to thyroid function and fertility, the final big factor that needs to be considered and investigated when someone has high levels of thyroid antibodies is primary ovarian insufficiency, also known as premature ovarian failure (POI). POI is a condition that affects the ovarian follicles that develop oocytes, an understandably critical component of getting pregnant. Research shows that POI occurs when the body either does not have enough follicles, or the eggs they do produce do not function properly, which can lead to ovulation issues and implantation failure.

POI is highly associated with the presence of elevated levels of thyroid antibodies. This can be measured via an AMH test (anti-müllerian hormone test).  In cases where test results show low levels of anti-müllerian hormones (AMH) and high levels of follicle stimulation hormones (FSH), this combination is often associated with POI and high levels of thyroid antibodies.

References:

Bridget Brady, MD, M. F. (2019). Thyroid Gland: Overview. Retrieved from Endocrineweb: https://www.endocrineweb.com/conditions/thyroid-nodules/thyroid-gland-controls-bodys-metabolism-how-it-works-symptoms-hyperthyroi

Hashimoto’s Disease. (2020). Retrieved from Cleveland Clinic: https://my.clevelandclinic.org/health/diseases/17665-hashimotos-disease#:~:text=Hashimoto’s%20disease%20affects%20the%20thyroid,and%20keep%20it%20working%20normally.