The disease that has plagued Missy Elliott for three years, reportedly causing mood swings, weight loss and — at its worst — the inability to drive or write, affects 1 in 30 people each year.

The thyroid gland is shaped like a butterfly and located in the front of the neck, just below the voice box or Adam’s apple. It controls metabolism and how fast the body runs. Heart rate, weight gain or loss, menstrual cycles, body temperature, muscle strength, and the intestines are all affected by thyroid hormones.

Someone who is hypothyroid — or not making enough thyroid hormone — may gain weight despite eating small amounts; have difficulty tolerating cold weather; feel sluggish or fatigued; or develop constipation, joint pain, heavy menstrual periods or even depression.

On the other hand, those with Graves’ disease, like Elliott, are hyperthyroid — making too much thyroid hormone. The problem with Graves’ is that the body’s immune system mistakenly attacks the person’s own thyroid, which causes the gland to release more hormone.

Classic symptoms of hyperthyroidism include feeling nervous, moody, jittery or tired; hand or muscle tremors; racing heartbeat, even at rest; diarrhea; sweating and unable to tolerate hot weather; hair loss; or losing weight despite eating more.

Graves’ disease, in particular, can also cause bulging eyes, thickened nails, or a goiter — where the thyroid swells and causes a large mass in the front of the neck.

There is no way to prevent Graves’ disease, and its cause is poorly understood. Some experts say that if someone already has the genetic setup for Graves’, any stress to the body could make the disease surface.

However, if diagnosed early, the patient has fewer years of suffering. It never goes away completely, but it can be controlled.

Graves’ runs in families. And, those with other autoimmune disorders such as childhood diabetes, rheumatoid arthritis, multiple sclerosis and lupus are more affected.

Women are more likely to develop Graves’ than men and, like Elliott, the diagnosis typically occurs between 30 and 40 years of age. Researchers have begun isolating genes for Graves’ that differ among races, but the disease appears to affect all racial groups.

For now, treatments include medication, radiation or surgery to calm down the overactive thyroid.