What is Hypothyroidism?

The thyroid is the engine of your body, says Dr. Jennifer Mammen, an endocrinologist and assistant professor of medicine at Johns Hopkins. While your fitness won’t affect how well that engine is or isn’t running, the thyroid sets a baseline for your metabolism and you’ll surely notice if that engine starts to slow.

“It’s an adaptive system,” says Dr. Mammen, “helping your body adapt appropriately to the environment and with what’s available.”

Hypo and hyperthyroidism occur when the thyroid is under and over performing respectively. Several elite athletes have recently shared their experience with hypothyroidism, specifically. In January 2022 Olympic marathoner Jared Ward shared his diagnosis of Hashimoto’s disease, which causes hypothyroidism.

Even more recently, Des Linden, the 2018 Boston Marathon champion and Olympian, revealed her hypothyroidism diagnosis in her newly released memoir Choosing to Run. Her situation prior to diagnosis was dramatic and dire. The nurse practitioner she saw informed her that her thyroid hormone levels were so low that she could possibly die before she could get an appointment to see a specialist for a second opinion. She needed to be on medication right away.

Linden’s experience is rare. While it’s estimated that up to 5 percent of the U.S. population has hypothyroidism, the life-threatening expression of myxedema coma–an extreme complication from abnormally low thyroid hormone involving difficulty breathing, confusion, extreme weakness, and low body temperature–has an incidence rate of 0.022 per 100,000 per year.

What does a typical hypothyroidism diagnosis look like? Here is what runners should know about when your body’s engine underperforms.

RELATED: Des Linden’s ‘Choosing to Run’ 

What is hypothyroidism?

Hypothyroidism is a condition in which the thyroid gland–a butterfly shaped gland located in the front of the throat–is underactive in producing hormones.

Thyroid stimulating hormone (TSH) is a pituitary hormone that triggers production of thyroxine (T4) and triiodothyronine (T3) from the thyroid gland. T4 and T3 are vital in major organ function, bone health, and setting your metabolic rate. Without enough of these hormones, as is the case in hypothyroidism, body functions slow down causing symptoms like fatigue, feeling excessively cold, muscle weakness, depression, and constipation, among others.

The symptoms of hypothyroidism are considered non-specific. There is no one symptom that is a giveaway among all diagnoses. Therefore, it requires a blood test–sometimes multiple–in order to diagnose.

According to the American Thyroid Association, women are one to eight times more likely than men to develop a thyroid condition. The reason why is unknown.

What causes hypothyroidism?

Running, when properly fueled, does not cause hypothyroidism, but there are still some elements of the condition that runners should be awa. Below are the common causes of hypothyroidism that runners should be aware of.

Primary hypothyroidism

In primary hypothyroidism, it is the thyroid gland itself that is not functioning properly, often caused by autoimmune diseases like Hashimoto’s thyroiditis or atrophic thyroiditis. In these cases, the immune system confuses thyroid gland cells as invaders and attacks them.

Secondary hypothyroidism 

In secondary hypothyroidism, the thyroid is functioning at a reduced capacity for a reason that is unrelated to the health of the gland.

RED-S and nutritional deficiencies related to running, for example, may cause thyroid function to be downregulated, but that is not necessarily diagnosed as hypothyroidism. “We call it secondary because it’s not the gland that has a problem or disease,” says Dr. Mammen.

She notes that the thyroid reacting to a lack of nutritional energy is an appropriate adaptation of the thyroid. When the nutritional deficiency is resolved, thyroid function picks back up again.

“The symptoms of thyroid disease are very nonspecific,  especially in a situation like an athlete, where there can be other contributing factors, like nutritional deficiencies,” she says.

Tertiary hypothyroidism

This very rare diagnosis involves an issue with the hypothalamus causing inadequate signals to the pituitary gland to send enough TSH to the thyroid.

When to see a doctor

If you feel any of the symptoms listed above, seeing a doctor is never a bad idea. If they suspect a problem with the thyroid, seeking out an endocrinologist, or a more specialized thyroidologist, can help you dial in a treatment plan.

Hypothyroidism is treated by supplementing with synthetic hormones. It may take a few tries to get the dosage correct, but it’s otherwise a simple treatment.

On the synthetic hormones, “physiology goes back to normal and athletic endurance and everything should return to normal,” says Dr. Mammen.

Dr. Mammen also points out that the synthetic hormone is not a miracle drug and should not be abused by people who do not have hypothyroidism. “There are conspiracy theories out there about thyroid hormone these days,” she says. “It’s the elixir of life kind of crowd.”

It’s something Linden herself struggled with after her diagnosis, not wanting to be lumped in with bad actors in sport who used the thyroid hormones for an unfair boost.

“I had to reconcile the fact that I wasn’t bulletproof, that I needed to get well, that taking medicine didn’t confer an advantage in the sense I had always thought of as cheating. It would simply treat the very real and serious symptoms. The realization was humbling and illuminating. My diagnosis made me take responsibility for my health,” she writes.

*Please note: This article is for educational purposes and should not be substituted for medical advice. Be sure to consult your physician or another licensed medical professional for a personalized treatment plan.