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What Are the Dangers of Too Much Thyroid Medication?

author image Helen Williams
Based in New York City, Helen Williams has been a feature writer for more than 15 years. She has written scientific posters, journal articles and book chapters for physicians, lawyers and the general public. Williams' narratives have appeared in publications including "Playgirl" and "Guideposts." She received a Bachelor of Arts in English from Tufts University.
What Are the Dangers of Too Much Thyroid Medication?
A pharmacist is holding a pill bottle. Photo Credit Thinkstock Images/Stockbyte/Getty Images


Many disorders of the thyroid, the gland that regulates the body's energy energy level, require medication, often for a lifetime. Too much thyroid medicine can cause serious side effects, including effects to the fetus in pregnant women. Thus, doctors must carefully monitor patients to ensure they are getting the correct dosage and that their thyroid hormone levels remain in the normal range.

Liver Damage

In the United States, two antithyroid drugs, methimazole and propylthiouracil, are used to prevent the overactive thyroid from making too much thyroxine, the thyroid hormone. Large doses of propylthiouracil and methimazole can seriously injure the liver, even to the point of death, reports MayoClinic.com. Because the risk is higher with prophylthiouracil, doctors taking recommend trying methimazole first, switching only if it proves intolerable.

Worsened Eye Symptoms

Radioactive iodine is sometimes used to treat Graves’ disease, the most common disorder of overactive thyroid, because it shrinks the thyroid gland and reduces excess hormone levels, reports MayoClinic.com. A problem, however, is that people may develop new or worsened eye problems, including bulging, dryness, tearing or redness. Although the effect is temporary, doctors avoid using radioactive iodine if eye symptoms are present.

Bone Loss

Levothyroxine is a synthetic hormone used to replace thyroid hormone due to underactive thyroid, such as Hashimoto’s disease, a condition where the immune system attacks the thyroid, reports MayoClinic.com. Too much levothyroxine can cause or worsen osteoporosis.

Heart Problems

Too much levothyroxine can also make the heart beat too fast or irregularly, explains MayoClinic.com. In patients with heart disease, doctors may begin with a small dose, gradually increasing it after careful monitoring of impact on the heart.

Adverse Effects on Fetus

Large doses of propylthiouracil or methimazole can cause inflamed thyroid or underactive thyroid in the fetus, according to Thyroid Disease and Pregnancy. Frequent monitoring of hormone levels and dosage adjustment in the pregnant mother reduces this risk.

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