The reported failure of serum TSH to rise inresponse to the low serum T3 ofsevere systemic illness may bedue to the known stress inhibition of TSH secretion. We thereforemeasured TSH and total and free thyroid hormones duringthe course of recovery from severe illness. During recovery, TSHincreased at a time when T3 was rising but still below normal(mean TSH during recovery, 6.5 ± 0.8 SEM εU/ml, n = 41vs. normal, 2. ± 0.2 SEM ±U/ml; n=31; P 3 andle s. r. strongly correlated with total T4 but notwith free T4. Average TSHconcentrations were also significantly elevated inseverely ill patients with hypothermi that was unrelated to coldexposure (mean TSH, 5.6 ± 1.3 ±U/ml; n = 11; P < 0.005). TheT3 concentrations in these sera were lower than those of otherseverely ill patient. Thus,during recovery from severe illness and during hypothermianot induced by cold, the relationship between serum T3and TSH is qualitatively similar to that seen in primary hypothyroidismand may imply a pituitar response to a deficiency ofthyroid hormone. (J Clin Endocrinol Metab54: 1229, 1982)