When we think about our menstrual cycles and fertility, we often think about the ovaries and the hormones estrogen and progesterone, the key players most of us are somewhat familiar with. However, these hormones are just two of many, which have various roles in the intricate dance of hormone balance that both regulates the menstrual cycle, and affects fertility. Thyroid hormone in particular plays a major role in these processes, and yet often gets overlooked when a woman is being worked up for menstrual cycle irregularities as well as fertility challenges.
Thyroid hormones are produced by the thyroid, a small, butterfly-shaped gland located at the front of the neck, towards the base. The thyroid gland primarily produces thyroxine (T4), the inactive form of thyroid hormone, which gets converted in the liver to its active form, triiodothyronine (T3). Every single cell in the body has receptors for T3, which binds to those receptors to exert various actions such as ramping up metabolism, regulating the storage and breakdown of glucose, lipids and proteins, affecting cardiovascular activity, digestion, hormone production and more.1
The irregular production of thyroid hormones due to conditions such as hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid) can lead to an imbalance in the sex hormones estrogen and progesterone, causing changes to menstruation, and by association, fertility.2
Hypothyroidism is incredibly common, affecting an estimated 1 in 8 women.3 Common symptoms of hypothyroidism include unexplained weight gain, fatigue, constipation, dry skin, brittle nails, hair loss, depression, cold intolerance and irregular periods.3 The effect on the menstrual cycle is due to a cascade of hormonal changes that occur when thyroid hormone production is low, and which typically results in the suppression of gonadotropin-releasing hormone (GnRH).4 GnRH is necessary for the production of follicle stimulating hormone (FSH) and luteinizing hormone (LH), which, if under-produced, can result in changes in ovulation and menstruation, ranging from heavy periods, irregular periods, or absent periods all together.4
Hyperthyroidism on the other hand is a bit less common, affecting an estimated 1 in 100 women.3 The symptoms of hyperthyroidism are essentially the opposite of what we see with hypothyroidism – unexplained weight loss, rapid heart rate, sweating, nervousness, irritability, diarrhea, and sleep disturbances which may also result in fatigue.3 In hyperthyroidism, excess thyroid hormone results in the over-production of GnRH.4 The effects of excess GnRH are not well understood at this time, but it may cause the body to produce too much testosterone and estrogen, resulting in menstrual cycle changes such as heavy periods or irregularly occurring periods.
Current guidelines actually don’t recommend regular screening for thyroid disease in women during routine visits the way blood pressure, cholesterol, and other measures are routinely screened, despite how common thyroid disease is among women. As a naturopathic doctor, I recommend screening for thyroid disease annually, and if your doctor is not doing this already, be sure to request it! In particular, if you experience irregular periods or any of the symptoms listed above, thyroid testing is an important piece of your workup that should not be overlooked.
Many doctors will only look at two measures when assessing the thyroid – thyroid stimulating hormone (TSH), and free T4. However, just looking at these two values does not give you the complete picture, as the process of thyroid hormone production, regulation, cellular uptake and use is another intricate process in and of itself. For this reason, I always recommend testing the following:
- Free T4
- Free T3
- Reverse T3
- Thyroglobulin antibodies (Tg Ab)
- Thyroid peroxidase antibodies (TPO Ab)
These additional tests are helpful in understanding whether the thyroid is being properly stimulated to make hormones, how well the body is converting inactive T4 to active T3, how well the body is utilizing T3, and whether an autoimmune process is affecting the thyroid, and to what degree.
An example I commonly see in my own practice is in women who have hypothyroidism that is considered “well-managed,” because their TSH levels are within a normal range despite experiencing ongoing symptoms of hypothyroidism, including menstrual cycle irregularities or infertility. Typically what we’ll see with further testing is that free T3 is still low, often due to issues like chronic stress or nutritional deficiencies. Because T3 is the active form of thyroid hormone, inadequate levels will result in ongoing symptoms of hypothyroidism. Even though TSH is normal, a functional hypothyroidism is still present, because thyroid hormone is insufficient.
So if you sense that something might be off with your thyroid, if you’re experiencing menstrual cycle irregularities, fertility challenges, or any of the above symptoms and haven’t had your thyroid checked, ask your doctor for a full thyroid work up!
- Sinha R, Yen PM. Cellular Action of Thyroid Hormone. [Updated 2018 Jun 20]. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK285568/
- Jacobson, M. H., Howards, P. P., Darrow, L. A., Meadows, J. W., Kesner, J. S., Spencer, J. B., Terrell, M. L., & Marcus, M. (2018). Thyroid hormones and menstrual cycle function in a longitudinal cohort of premenopausal women. Paediatric and perinatal epidemiology, 32(3), 225–234. https://doi.org/10.1111/ppe.12462
- General Information/Press Room. American Thyroid Association. (n.d.). Retrieved March 30, 2022, from https://www.thyroid.org/media-main/press-room/
- Shomon, M. (2021, November 18). How Thyroid Function Affects Menstruation. Verywell Health. Retrieved March 30, 2022, from https://www.verywellhealth.com/menstrual-problems-and-thyroid-disease-3231765