Underactive

Underactive is an adjective used to describe a system or process that operates at a lower activity level than normal. In biomedical contexts, underactive typically refers to organs, glands, or bodily processes that produce insufficient hormones or function poorly relative to standard expectations. The most common usage is in relation to the thyroid gland: an underactive thyroid, or hypothyroidism, occurs when thyroid hormone production is reduced. Symptoms can include fatigue, weight gain, sensitivity to cold, dry skin, constipation, and slowed heart rate. Other examples include underactive bladder, which describes reduced detrusor muscle activity leading to incomplete emptying or urinary retention, and adrenal underactivity, where cortisol production is insufficient.

Causes of underactivity vary by organ and may include autoimmune disease, iodine deficiency, medications, aging, or injury. In some cases, underlying conditions such as pituitary dysfunction or genetic factors contribute to reduced activity of a target gland.

Diagnosis generally relies on symptom assessment in combination with laboratory tests or functional studies. For hypothyroidism, blood tests showing elevated thyroid-stimulating hormone (TSH) and low free thyroxine (T4) support the diagnosis; for adrenal underactivity, cortisol measurements and stimulation tests are used; urodynamic testing may assess detrusor function for bladder underactivity.

Treatment depends on the cause and tissue involved. Hormone replacement therapy is common, such as levothyroxine for hypothyroidism or hydrocortisone for adrenal insufficiency. Management of underactive bladder may include bladder training, intermittent catheterization, or medications. Prognosis improves with appropriate diagnosis and ongoing management, though some conditions require lifelong treatment.

Etymology: underactive combines the prefix under- meaning below or less than, with active meaning having activity or function.