Ultrasensitive assay of thyroid stimulating hormone in patients with acute non-thyroidal illness

Serum TSH in critically ill euthyroid patients is generally within the normal range when measured with conventional radioimmunoassays. Sensitive immunoradiometric assays allow detection of low levels of serum TSH. We assessed this method in a prospective study of 34 euthyroid patients admitted to our critical care unit. Serum TSH ranged from 0.12 to 3.60 mU/l and was significantly lower for the whole group than in the controls (P less than 0.001), as also were serum total T4 and T3 values (P less than 0.001). However, 21 patients had a serum TSH within the normal range (group 1) and 13 patients (33%) had a serum TSH less than 0.40 mU/l (group 2). The two groups did not differ in age, sex, type and severity of illness, outcome, and serum T4 and T3 levels. However, the magnitude of TSH increase from the baseline value after the i.v. injection of 200 micrograms of TRH, assessed by the 30 min TSH/basal TSH ratio was significantly higher in group 2 (P less than 0.05). These results suggest that a substantial proportion of patients with acute illness have a clearly low serum TSH, unaccounted for by age, sex, type or quantified severity of illness, serum T4 and T3 levels. In addition, the secretory capacity of the pituitary to exogeneous TRH is significantly enhanced in those patients with low basal TSH.