Thyroid test abnormalities in traumatic brain Injury: Correlation with neurologic impairment and sympathetic nervous system activation

Abstract

Acute illness is well known to affect thyroid function, but there are few studies correlating the severity of the underlying medical problem with indexes of thyroid function and little is known about its cause. Traumatically brain-injured patients were selected because they were a relatively homogeneous, previously healthy group with a condition whose severity was readily quantifiable. In 66 such patients, the relationships between changes in thyroid function tests (thyroxine, free thyroxine, triiodothyronine, reverse triiodothyronine, and thyrotropin levels), catecholamine and cortisol concentrations measured on admission and again four days after the accident, and neurologic function assessed by the Glasgow Coma Score (GCS) were studied. Triiodothyronine and thyroxine levels fell significantly within 24 hours of injury. Four days after the accident, patients with the greatest neurologic dysfunction had the lowest triiodothyronine and thyroxine levels; significant correlations were present between the Day 4 GCS and concomitant thyroxine ( r = 0.47, p <0.0001), free thyroxine ( r = 0.32, p <0.02), and triiodothyronine ( r = 0.50, p <0.0001) levels. Reverse triiodothyronine values remained unchanged throughout the study even in the most severely affected patients; the rise in thyrotropin levels was not significant (1.2 ± 0.2 to 1.7 ± 0.3 μU/ml, p = NS). Patients who died or remained vegetative had thyroxine and triiodothyronine levels 30 percent to 50 percent lower than those who had a good recovery (p <0.05). Highly significant correlations were present between Day 4 thyroxine and triiodothyronine levels and admission and Day 4 norepinephrine and epinephrine concentrations. There was no association between admission or concomitant cortisol levels and thyroid function on Day 4; treatment with high-dose dexamethasone did not influence these indexes. Thus, patients with traumatic brain injury exhibit a gradient of thyroid dysfunction that occurs promptly, is dependent upon the degree of neurologic impairment, and reflects ultimate outcome. The significant association with catecholamine levels suggests a role for sympathetic nervous system activation in its causation, independent of a generalized stress response, since there is no correlation of thyroid test abnormality with the degree of adrenocortical secretion.