underactive       
adjective uk/ˌʌn.dərˈæk.tɪv/ us/ˌʌn.dɚˈæk.tɪv/
 不够活跃的,缺乏活力的
(of part of the body) not working as much or as effectively as it should
  About 27 million Americans have an overactive or underactive thyroid (= an organ in the front of the neck). 大约有2700万美国人甲状腺过度活跃或机能低下。 
 This part of the brain has been shown to be underactive in people who are depressed. 研究表明,抑郁症患者大脑的这一部分活动不足。 
 - More examples
 - If your thyroid is underactive, you can gain weight.
 - I have to take all kinds of hormone replacements because my thyroid gland is underactive.
 - Using a new brain imaging technique, they have identified a key area of the brain that is underactive in children with attention deficit hyperactivity disorder (ADHD).
 - For an underactive bladder, the patient may be placed on a bladder routine with fluid intake and output adjusted.
 - People may have an underactive immune system due to a condition present from birth, certain diseases, or treatment with certain medicines (eg immunosuppressants, chemotherapy).