While your legs and cardiovascular system are clearly the primary contributors to your ability to run, being in top condition depends on your whole body, including your endocrine system. This system is responsible for producing hormones, many of which -- notably insulin, testosterone, growth hormone and thyroid hormone -- play a direct role in how you feel and perform while running. Disturbances to your thyroid can profoundly affect your exercise and overall well-being.

What the Thyroid Does

The thyroid is a butterfly-shaped gland in the front of your neck. The active form of hormone it produces, called thyroxine or simply T4, participates in processes related to development, growth and metabolism. According to Colorado State University, T4 probably affects every type of cell in the body 1. Higher T4 levels imply a higher metabolic rate in all respects -- increased heart rate, and greater breakdown of fats and carbohydrates -- and are associated with greater mental alertness. Very high levels result in anxious states.

Hyperthyroidism and Running

I Don't Lose Weight If I Eat 1200 Calories a Day

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If you are a distance runner, you may be trying to lose weight or are accustomed to being very thin, so you may be more likely than others to miss these symptoms. Often, physical signs such as a goiter, or lump in the throat, or protruding eyeballs, are present. Graves' disease is a very common cause of hyerthyroidism. If your weight is dropping despite no change in training or eating habits, see your doctor for a blood test.

Hypothyroidism and Running

As you might guess, the symptoms of hypothyroidism are opposite those of hyperthyroidism: sluggishness, weight gain, feeling cold even in warm environments and depression. Left untreated, it can cause serious health problems over time. An underactive thyroid is usually the result of an autoimmune reaction. According to Pete Magill of "Running Times" magazine, runners with a family history of thyroid problems may induce an autoimmune attack on their own thyroid gland by training heavily. The end result in distance runners is an inevitable decline in motivation and performance.

Long-Term Outlook

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The key to treating thyroid problems is identifying them early. This done, treatment is straightforward and symptom relief is virtually immediate. An overactive thyroid is treated using surgery or radioactive iodine, after which you will have to take T4 supplements for the rest of your life. Hypothyroidism is treated with synthetic T4 as well, and it may take time to get the dose right. In either case, once your weight and mood are stable and you are feeling better, you should be able to return to your running routine without any lasting consequences.