insufficiently active: an underactive thyroid gland.

1955–60; under- + active

underactivity, underactiveness, noun
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World English Dictionary
underactive (ˌʌndərˈæktɪv)
(of the thyroid or adrenal glands) not functioning at full capacity

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
The technique excites nerve cells in a small area of the brain that is
  underactive in depressed patients.
They engineered cells to become either hyperactive or underactive in response
  to specific wavelengths of light.
Pale skin areas are due to too little melanin or underactive melanocytes.
The immune response may be overactive or underactive.
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