Hypothyroidism and the Endurance Athlete

ankle sprain, painAs runners, we perform more cardio than an average exerciser–and most of us can’t imagine a life without running. However, as with all forms of exercise, we need to be cognizant of the challenges that come with extreme amounts of exercise.

Researchers have discovered that heavy amounts of cardiovascular exercise causes the body to stay in flight of “flight or fight mode” and this can cause it to lower the production of the thyroid hormone called T3. This can lead to hypothyroidism.


Symptoms include:

Weight gain


Sensitivity to cold



Joint or muscle pain

Paleness or dry skin

In a study published in Neuroendocrinology Letters,  acute aerobic exercise on thyroid hormones was researched in 60 male well-trained athletes by performing bicycle ergometer at 45% (low intensity), 70% (moderate intensity), and 90% (high intensity). At each intensity level, heart rate, blood lactic acid, T3 (as mentioned above), T4, free triiodothyronine (fT3) and thyroid stimulating hormone (TSH) values were measured.

Results showed that exercise at the 70% level, where you probably do your tempo runs, caused the most changes in the hormone values. Even at 90% intensity, where you do your track/speed workouts, the levels of T3 and T4 started to fall. Researchers concluded “maximal aerobic exercise greatly affects the level of circulating thyroid hormones.”

Another research study looked at females who ran 14 miles per week, but asked them to up it to 30 miles per week. Results showed again that an increase in cardiovascular training had negative effects on hormone levels.

This does not mean you are going to suffer from hypothyroidism because you run–often the body will adjust to the new training on its own. But it is something to be aware of and watch for any of the above symptoms, which can often be misinterpreted as simply over training.

The best way to be diagnosed with hypothyroidism is with blood tests and it can be treated with medications to help balance the thyroid.

The best athlete is the one who is informed!