EFFECT OF SEVERE, CHRONIC ILLNESS ON THYROID FUNCTION

Abstract

Most clinically euthyroid patients with severe, chronic, non-thyroidal illnesses (i.e., " sick euthyroid " patients) have abnormal thyroid-function tests, and consequently assessment of their thyroidal status may be very difficult. The most striking abnormality detected in 75 sick euthyroid patients was a highly significant reduction in the mean total serum-triiodothyronine (T3) levels, with most patients having total T3 levels in the hypothyroid range. The severity of the illness correlated well with the reduction in total serum-T3 levels. The mean absolute free serum-T3 concentration was significantly lower than in the control patients. The mean total serum-thyroxine (T4) levels were also significantly reduced, although, unlike the total serum-T3 levels, they remained within the normal range. The total serum-T4/T3 ratios were generally higher in the sick euthyroid patients compared with the controls. The absolute free serum-T4 concentrations (as measured by the effective-thyroxine-ratio test) were normal, and serum-T.S.H. was not increased in any patient. The mean effective binding capacity of thyroid binding proteins in 11 sick euthyroid patients and the control patients was not significantly different. 3 patients had normal T.S.H. and T3 responses to oral thyrotrophin-releasing hormone (T.R.H.) and 2 other patients had normal T3 responses to intramuscular T.S.H. The major cause of the low total and absolute free serum-T3 concentrations in the sick euthyroid patients seemed to be inhibition of extrathyroidal conversion of T4 to T3. Decreased concentrations of binding proteins are unlikely to contribute greatly to the abnormalities of thyroid-function tests in the patients studied. Depressed production of T.R.H., secondary to stress or undernutrition consequent upon chronic illness, may also contribute by reducing thyroidal hormone output.