Definition of "underactive" []

• Less than normally active. (adjective)

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• "And finally, a medical condition such as underactive thyroid could cause a significant drop in metabolism, so if you have noticed increased fatigue, hair loss, constipation, weight gain or dry skin, you may want to talk to your doctor about getting a simple blood test to make sure that your thyroid is not underactive."
• "They have overactive inner critics and underactive inner supports."
• "In severe cases an underactive thyroid can be treated with hormone therapy and an overactive thyroid with medication, radioactive iodine, or surgery."