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  1. insufficiently active: an underactive thyroid gland.
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Origin of underactive

First recorded in 1955–60; under- + active
Related formsun·der·ac·tiv·i·ty, un·der·ac·tive·ness, noun
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for underactive

Historical Examples

  • An underactive thyroid, known as hypothyroidism, is caused by an inadequate production of thyroid hormone.

    When You Don't Know Where to Turn

    Steven J. Bartlett

British Dictionary definitions for underactive


  1. (of the thyroid or adrenal glands) not functioning at full capacity
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012