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If your underactive thyroid is treated, you should be able to lead a normal, active life. But if it's not treated, your symptoms may gradually get worse.

If your underactive thyroid isn't treated, your symptoms may get gradually worse.

You may not be able to think as clearly or as fast. You may get depressed, gain weight or become constipated, or your skin may become very dry. Your voice could get hoarse. If you are a woman, your periods may become irregular. This could affect whether you can get pregnant.

An underactive thyroid can make your heart beat more slowly and your cholesterol level rise.¹ Although researchers aren't sure, both of these things may put you at risk of getting heart disease.

Very occasionally, an underactive thyroid can cause a more serious illness that needs treating in the hospital.² These illnesses include:

• A very low body temperature (hypothermia)
• Heart failure
• Fluid around the lungs or heart
• An obstruction in the gut
• Serious mental health problems such as psychosis (being out of touch with reality) or dementia.

If you have the mild form of this condition and don't get any symptoms, you could go on to get the more serious form in time. Each year between 2 and 4 out of 100 women with mild hypothyroidism go on to get symptoms.³ The risk is much higher in men.³

Although researchers aren't sure, you may also be more likely to get heart disease. One study found that people with a mildly underactive thyroid had higher levels of cholesterol in their blood.⁴

Another study showed that elderly women with a mildly underactive thyroid were more likely to have a heart attack and to have fatty deposits in the blood vessels of their heart.⁵

But we're not sure whether getting treatment for a slightly underactive thyroid gland reduces your risk of heart disease. Some research shows it may help your heart to work better.⁶

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.