Thyroid hormone changes during critical nonthyroidal illness

During critical illness, serum triiodothyronine (T₃), thyroxine (T₄) and thyroid stimulating hormone (TSH) concentrations often fall outside (usually below) the normal reference ranges. In the absence of clinical features of a thyroid disorder, abnormal serum T₃, T₄ and TSH concentrations are usually a transient part of the complex endocrine response to critical illness, and rarely reflect persistent thyroid disease. Do not start treatment for a thyroid disorder during critical illness based on thyroid function test results alone.

During critical illness, a common finding is a low serum TSH concentration with a low serum T₄ concentration, which otherwise suggests hypothyroidism secondary to hypopituitarism; however, a persistent pituitary abnormality is rare. These changes are usually caused by transient central inhibition of TSH secretion (either spontaneous or caused by drugs), altered deiodination, and accelerated clearance of thyroid hormones related to critical illness.

High serum TSH concentration with normal or low serum T₄ concentration, which usually indicates primary hypothyroidism, can occur transiently during recovery from critical illness.

Many drugs can influence thyroid function; these effects can be amplified during critical illness. Patients who are critically ill are also likely to be taking multiple drugs, so always consider the effect of drugs on thyroid function. See Drugs that influence thyroid hormones.

In a patient who had abnormal serum thyroid hormone concentrations during a critical illness, assess for persistent thyroid disease by repeating thyroid function tests around 6 weeks after recovery.