CHICAGO – Hypothyroid patients exhibit objective cognitive deficits and motor slowing rendering them unsafe to operate a motor vehicle.
That’s the key take-home message from a longitudinal study in which 32 patients with thyroid cancer completed an extensive battery of neurocognitive and psychological tests as well as measured performance on a driving simulator at three time points: while euthyroid, again while temporarily hypothyroid as part of their cancer therapy and assessment, and finally while once again euthyroid after restoration of thyroid hormone therapy.
"These findings provide objective evidence warranting admonitions against operating motor vehicles for hypothyroid patients and confidence in removing such stipulations upon restoration of a euthyroid state," Dr. Kenneth B. Ain said at the joint meeting of the International Congress of Endocrinology and the Endocrine Society*.
In his own clinical practice he has long included a boxed warning against driving while hypothyroid on all of his written instructions to patients. But most physicians don’t warn their hypothyroid patients that they are driving impaired, nor do the joint practice guidelines of the American Thyroid Association and American Association of Clinical Endocrinologists address the issue. That’s largely because there hasn’t been objective, quantitative evidence to provide firm support for such cautionary admonitions – until now, observed Dr. Ain, professor of medicine and director of the thyroid oncology program at the University of Kentucky, Lexington.
While hypothyroid, study participants experienced an 8.5% increase in braking time on a driving simulator. That’s equivalent to the degree of impairment other investigators have shown to be associated with a blood alcohol level of 82 mg/dL, which is above the legal driving limit in the United States.
"Once our study is published, a patient who is involved in an auto accident [in which] there is death or significant harm could be considered an impaired patient. And if physicians do not warn the patient of this risk, they would be considered an agent of harm. They could be liable for the consequences, the same as a neurologist who doesn’t warn a patient with a grand mal seizure disorder not to drive," Dr. Ain said.
In an interview, he noted that thyroid cancer patients undergoing thyroid hormone depletion temporarily as part of their treatment are merely a small fraction of the total impaired hypothyroid driver population. Investigators with the Framingham Heart Study have reported that 4.4% of individuals above age 60 are hypothyroid. Many of these individuals remain undiagnosed or undertreated. Plus, the noncompliance rate with levothyroxine therapy has been estimated at 17%-32%.
Moreover, once a patient is newly diagnosed as being profoundly hypothyroid and receives a prescription for thyroid hormone replacement, there is a lag time involved in achieving a euthyroid state. The half-life of levothyroxine is 1 week. It takes 6-8 weeks to reach a steady state. Probably at least 2 weeks of therapy are required before there is any improvement in the neurologic impairments documented in this study, Dr. Ain speculated.
"We’re really talking here about a public health problem, one that requires a public health response and acknowledgment that this is a danger," according to the endocrinologist.
Testing during the hypothyroid phase of the study showed significant declines in measures of executive function and information-processing speed. Fine motor performance of the hands was slowed by 13%. Mean scores on the Beck Depression Inventory deteriorated from 7.9 while euthyroid to 18.9 while hypothyroid, consistent with mild bordering on moderate depression; this depression was characterized by vegetative symptoms and altered mood, but without the impaired self-esteem and sense of guilt often characteristic of other forms of depression.
Dr. Ain reported receiving a research grant from Genzyme, which funded this study.
*Correction, 7/1/2014: An earlier version of this article misstated the name of the International Congress of Endocrinology.