More than 27 million Americans have either an overactive or underactive thyroid, according to the American Association of Clinical Endocrinologists, but only half of them know it.
The brain tells the gland the level of hormone to produce, but if the thyroid is sick, then it won't make enough and it grows bigger with scar tissue, Oz said. The gland could create swelling in the neck, he added.
Click here to take a thyroid "neck check" test from the American Association of Clinical Endocrinologists.
Women are seven times more likely than men to have thyroid problems, Oz said.
"You should never be able to see the thyroid gland normally," Oz said.
So how can you tell if something is wrong with your thyroid gland? Oz compared the thyroid gland to the thermostat in a house, saying that you don't always know if the thermostat is set to the correct temperature, but you don't feel well if it's not.
There are two types of thyroid disorders: hypothyroid and hyperthyroid.
Hypothyroid is when the body does not produce enough hormone, in other words when the thermostat is too far down, Oz said.
Symptoms of hypothyroid include feeling cold, fatigue, unexplained weight gain, depression and brittle fingernails and hair.
Hyperthyroid is when the thermostat is turned up too high. Symptoms include a sensitivity to heat -- being "uncomfortabley hot," Oz said -- sudden weight loss, rapid or irregular heartbeat, anxiety attacks and difficulty sleeping, Oz said.
Oz explained that thyroid disease is caused by exposure to radiation, smoking and autoimmune disease.
Medications such as synthroid or armour thyroid will also help treat low thyroid function, Oz said, and there are also foods that can help.
"Food is also a stimulator of thyroid hormone," Oz said.
If you have hyperthyroid, adding soybean-related foods and cruciferous vegetables, such as broccoli, cauliflower or Brussels sprouts, to your diet can help slow down the gland.
For hypothyroid, Oz said steaming the cruciferous vegetables and cooking tofu will help.
It's also important to take Vitamin D.
But Oz cautioned that even if you are on medication, don't assume that you are being treated adequately -- talk to your doctor about finding the right thyroid level for you.
"If you've got symptoms, even if the blood sympotoms come back borderline normal," you might need treatment.
It's about "symptom management, not getting the right blood test," Oz said.
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