Pituitary-thyroid dysfunction in severe non-thyroidal disease: "low-T4 syndrome".

Thyroid function tests were obtained from 335 consecutive patients admitted to an intensive care unit. Twenty patients suffering from severe non-endocrine diseases (septicaemia, fulminant hepatic and renal failure, acute pancreatitis, polytrauma, cerebral haemorrhage) were found to have serum thyroxine levels in the hypothyroid range (less than 4 micrograms/dl). Serum concentrations of total thyroxine (2.3 +/- 0.2 micrograms/dl), triiodothyronine (0.23 +/- 0.03 ng/ml), and thyroxine binding globulin (15.1 +/- 1.3 micrograms/ml) were reduced, but were above normal for reverse triiodothyronine (0.43 +/- 0.06 ng/ml). Response of TSH secretion to iv TRH was found to be either normal, lowered or absent. Primary hypothyroidism was excluded, as no enhanced TSH response was observed in any case. Although decreased thyroxine levels may be due to increased thyroid hormone degradation it appears that associated impaired TSH responsiveness to TRH may result from illness-related inhibition of pituitary TSH release. Although the finding of decreased thyroid hormone levels is not rare in care patients, it represents an index of poor prognosis. Differentiation between this "low-T4 syndrome" and true hypothyroidism depends essentially on clinical symptoms and course of disease.