The relationship between serum triiodothyronine and thyrotropin during systemic illness.

The reported failure of serum TSH to rise in response to the low serum T3 of severe systemic illness may be due to the known stress inhibition of TSH secretion. We therefore measured TSH and total and free thyroid hormones during the course of recovery from severe illness. During recovery, TSH increased at a time when T3 was rising but still below normal (mean TSH during recovery, 6.5 +/- 0.8 SEM microU/ml, n = 41 vs. normal, 2.5 +/- 0.2 SEM microU/ml; n = 31; P less than 0.001), TSH concentrations were negatively correlated with total and free T3 and less strongly correlated with total T4 but not with free T4. Average TSH concentrations were also significantly elevated in severely ill patients with hypothermia that was unrelated to cold exposure (mean TSH, 5.6 +/- 1.3 microU/ml; n = 11; P less than 0.005). The T3 concentrations in these sera were lower than those of other severely ill patients. Thus, during recovery from severe illness and during hypothermia not induced by cold, the relationship between serum T3 and TSH is qualitatively similar to that seen in primary hypothyroidism and may imply a pituitary response to a deficiency of thyroid hormone.