Description

In non-thyroidal illness abnormalities in thyroid function tests are frequently observed. Plasma TSH concentrations may be suppressed in the acute phase and on recovery TSH concentrations may rise transiently into the hypothyroid range. Total T3 concentrations fall as a result of impaired hepatic uptake of T4 and conversion of T4 to T3. Plasma concentrations of reverse T3 increase due to impaired clearance not increased production. Illnesses modifies both the concentration and binding capacity of the plasma thyroid hormone binding proteins, this in turn tends to diminish the total thyroid hormone concentrations in plasma and raise the free hormone fraction. When measured by a reliable method free T4 concentrations usually remain within the reference range or are modestly raised in the sick patient. Some recent reports have suggested that increased freeT3 concentrations can be found in illness though most studies suggest low concentrations are found.