What is hypothyroidism?
Hypothyroidism is a condition where the thyroid gland is underactive. This means the gland does not produce enough of the thyroid hormone - this can affect the rest of the body. Common causes of hypothyroidism are autoimmune diseases, such as Hashimoto’s thyroiditis, surgical removal of the thyroid, and radiation treatment.
Can I use Natural Cycles, and how will it affect my temperature?
Those with hypothyroidism can still use Natural Cycles to monitor their fertility. This condition will lead to very fluctuating temperatures if untreated, and users of the app may receive messages about their temperature changes. Keep a close eye on your temperatures if you have hypothyroidism, as regularly fluctuating temperatures may cause the app to struggle with detecting your ovulation. Please note this doesn’t make the app any less effective, but you may receive more red days if it struggles to detect ovulation.
If you are treating your hypothyroidism, your temperature will most likely be a bit colder than the average (this is roughly between 35.8-36.4°C, or 96.44 - 97.52°F, during the follicular phase). However, this won’t stop the app from detecting your ovulation. Keep an eye on the Graph view to monitor your temperature fluctuation - the temperature increase after ovulation should still occur, and the algorithm should still be able to find your fertile and non-fertile days when you are taking medication for hypothyroidism.
Will I ovulate?
If you are taking medication for your hypothyroidism, you will most likely ovulate and still get green days. You may experience irregular cycles; these cause more red days since the app has to provide a safety buffer for shifting ovulation day. The algorithm is designed to minimize risk, so if there’s a chance you may be fertile, you’ll be given a red day.
Partial source: https://www.thyroid.org/hypothyroidism/