The main aims of the thesis were as follows: (1) To determine the ranges of total thyroxine, triiodothyrone, TSH and cortisol concentrations at different levels of illness severity, and to relate these to mortality; (2) To determine the range of responses to standard dynamic tests of adrenocortical and thyroid function, and relate them to severity of illness and mortality; (3) To determine the relationship between the extent of the changes in total thyroid hormone concentrations during illness and metabolic rate; (4) To accurately determine the prognostic value of measurement of thyroxine, triiodothyrone, TSH and cortisol on admission to an ICU. The majority of critically ill patients had plasma cortisol concentrations above the upper limit of normal in health. In keeping with Seyle's General Adaption Theory, plasma cortisol concentration correlated with severity of illness. The normal ranges of plasma cortisol concentration, defined as population mean +/- 2SD, differed according to the severity of illness. A plasma cortisol concentration above 200 nmol/L would be within the 95% range for a moderately ill patient (APACHE II score <16) whereas a concentration of under 400 nmol/L would be below the expected range in a severely ill patient (APACHE II score >24). A prognostic index based on admission measurements of cortisol, thyroxine and thyrotropin concentrations was developed using multiple logistic regression analysis. The model obtained predicted outcome of illness with significantly greater accuracy than APACHE II scores.