Hypothyroidism may lead to impaired driving

Hypothyroidism, insufficient thyroid hormone, is very common and has been known to cause impairment of many bodily functions, including brain function. Until now, studies have not sufficiently explored the extent of brain impairment and whether hypothyroid people are safe drivers, said the study's senior investigator, Kenneth Ain, MD, from the University of Kentucky and the Veterans Affairs Medical Center in Lexington, KY.

"We found that hypothyroid patients being tested on a driving simulator had a similar performance to that of drivers with a blood alcohol level above the legal limit in the U.S.," said co-author Charles Smith, MD, also of the University of Kentucky. "Physicians should warn their hypothyroid patients to avoid driving until they have been sufficiently treated with thyroid hormone."

In this study, thirty two patients with thyroid cancer, who were undergoing preparation for radioactive iodine scanning by stopping thyroid hormone, were evaluated with a battery of neurological and psychological tests, as well as testing on a driving simulator. They were studied when they were taking thyroid hormone, again when they were off of thyroid hormone, and then finally when they were back on thyroid hormone therapy. Hypothyroid patients had depression and also showed declines in neurological function that resulted in increased automobile braking times; similar to the performance of drivers with a blood alcohol level of 0.082 g/100 mL. Taking thyroid hormone reversed all of these effects.

"Our results uncover a potential public and personal health hazard regarding impaired hypothyroid drivers," Ain said.