Although up to 90% of intensive care unit (ICU) patients have been found to have abnormal thyroid function tests (most commonly low triiodothyronine [T₃]), the vast majority of them are found to have sick euthyroid syndrome, also known as nonthyroidal illness syndrome. Because of this complicating factor it has been very difficult to quantitate the true incidence of hypothyroidism in ICU patients, but estimates range from 5% to 30% of patients admitted to medical ICUs.

Signs and symptoms of hypothyroidism exist along a continuum. Patients may exhibit some symptoms while lacking others, and the most severe symptoms are present only when a patient has progressed to myxedemacoma. Possible manifestations of hypothyroidism include hypoglycemia, hypothermia (or cold intolerance), hypotension, hypoventilation, hyponatremia, bradycardia, electrocardiogram changes (T-wave inversions and eventually J waves), gastrointestinal atony, bladder atony, skeletal muscle myopathy, doughy nonpitting edema, fatigue, lethargy, and altered level of consciousness. Perhaps the most ubiquitous sign of hypothyroidism is a pericardial effusion, which is present in up to 30% of hypothyroid patients; pleural and peritoneal effusions can be seen as well. Medications that have been associated with hypothyroidism include lithium, amiodarone, aminoglutethimide, interferon alpha, thalidomide, betaroxine, and stavudine.

Critically ill patients who demonstrate any number of the aforementioned stigmata of hypothyroidism should be tested for thyroid function as early as possible as this minimizes the likelihood of values being abnormal because of sick euthyroid syndrome. There are many tests available for screening and diagnosis of thyroid dysfunction and
some controversy surrounds the order in which these tests should be completed. The most common method, which provides >90% sensitivity for detection of hypothyroidism, is a combination of thyrotropin and either free thyroxine (T₄) or total T₄ and free T₄ index. In primary hypothyroidism (which accounts for 95% of hypothyroidism), thyrotropin levels will be elevated outside the normal range of 0.5 to 5.0 mU/L and in most cases of clinical hypothyroidism will actually be elevated above 20 mU/L. In secondary, or central, hypothyroidism, which is due to adenohypophyseal or pituitary dysfunction, the thyrotropin will be very low or undetectable. In either situation, free T₄ will be below the normal range. Total T₄ levels are not helpful because only free hormone is active and total T₄ levels may be low in the face of normal free T₄ in situations of altered protein binding or decreased proteins available for binding. The free T₄ index takes into account binding and protein availability by multiplying the total T₄ by a factor determined by T₃ resin uptake.