Maybe you’ve heard of the “butterfly effect:” the theory that even tiny changes can have big consequences, such as the distant flapping of a butterfly’s wings later causing a hurricane. That idea also applies to the thyroid, a gland in the neck that happens to be butterfly shaped. It’s small, but its role in the body is huge.

Thyroid hormone (thyroxine) controls how quickly each of our cells uses energy and is vital to cell health. It also regulates all of our metabolic systems: those controlling body fat, bone composition and energy. From the very beginning, it’s involved in our neuronal development, which means that it infuences our nervous system, cognition and thinking.

In young people, it influences growth. A poorly functioning thyroid gland can even stunt a child's intellectual development. When the thyroid is even a little bit out of a whack, the effects can be dramatic—it can cause a hurricane. An off-kilter thyroid isn’t always easy to identify, however. That’s because the thyroid has an impact on so many of the body’s systems that the symptoms can be wide-ranging.

There are two types of thyroid imbalances. A person who’s producing too much thyroid hormone is said to have hyperthyroidism. While this condition is very serious, it’s comparatively rare— only about one to two percent of the population has it. We’ll discuss hyperthyroidism in more detail subsequently.

We’re focusing first on underactive thyroid, or hypothyroidism, which is far more common, affecting at least 4 to 5 percent of people, or possibly more—maybe even as much as 8 to 10 percent of us. That may be why thyroid hormone, levothyroxine, has been one of the most commonly prescribed drugs in the country for years.