In order to study the effects of a severe bacterial infection on thyroid function rats were subjected to group A streptococcal pneumonia, and thyroid activity determined by measurement of the rate of discharge of I131 from the thyroid gland.
Decreased thyroid activity of moderate to marked degree was observed in the majority of infected animals. Infected animals ate less than normal animals. Since fasting leads to decreased thyroid function, I131 release rates were measured in control animals pair-fed with infected animals and in control and infected animals force-fed a normal intake. The reduction in thyroid activity seen in acute infection was found to be partly but not entirely due to the associated voluntary food restriction.
Although the adrenal glands of rats dying of pneumonia were very large, the thyroid inhibition occurring during the course of experimental pneumonia was not secondary to increased adrenocortical function since infected, adrenalectomized animals, receiving injections of cortisone, showed thyroid inhibition comparable to that observed in intact infected animals.
The possible influence of the level of thyroid activity on resistance was evaluated. In two experiments a total of 80 rats in 3 groups was pretreated with propylthiouracil and injected daily with 3 dosage levels of 7-thyroxine for 3 weeks prior to inoculation. Under these conditions mortality rates in infected animals fell with decreasing levels of thyroid function. However, induced hypothyroidism was found to afford no protective effect in comparison with untreated infected animals.