Researchers from the University of Kentucky looked at whether significant hypothyroidism could impair driving ability. They studied 32 patients who had thyroid cancer - and who were taken off their thyroid hormone drugs and became hypothyroid prior to radioactive iodine scanning to detect cancer recurrence.

They had the patients do a driving simulator test, and according to study co-author Dr. Charles Smith, "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. Physicians should warn their hypothyroid patients to avoid driving until they have been sufficiently treated with thyroid hormone."

According to study co-author Dr. Ken Ain, "These patients had delayed reaction times, and their response in terms of braking their vehicles was as bad as those above the legal limit for alcohol intoxication."

Said Ain: "Nobody has truly studied the changes that take place in hypothyroidism...we routinely warn patients that they should not be operating motor vehicles while hypothyroid. We undertook a study, with the funding of Genzyme, to evaluate the driving ability and the neurologic function of patients who were made hypothyroid for thyroid cancer radioiodine therapy by thyroid medicine withdrawal."

Dr. Ain also referenced the Colorado Thyroid Prevalence Study of 2000, which found that almost 10% of the study population was hypothyroid and not diagnosed, suggesting that this group may also be a driving danger.

Are You Safe To Drive?

If you are hypothyroid, don't panic.

The question is, at what level of hypothyroidism is the typical person impaired? The answer is not clear, as the study results that have been released did not specify the actual TSH levels of the more than 30 thyroid cancer patients in the study. Typically, however, within a few weeks of stopping their thyroid medication, thyroid cancer patients (who almost always have had their thyroid glands surgically removed) see a rapid rise in TSH levels to 20.0, 30.0 and higher. So "significant" hypothyroidism, by the definition of this study, does not appear to be talking about mild hypothyroidism.

If you have had an elevated TSH level in the past, and noticed that your concentration, reflexes, responses, or focus were affected, you should, however, be careful about driving until your treatment is balanced, consistent and effective.

But the bottom line? If your hypothyroidism treatment is optimized, you should be safe to drive.

The Study's Funding

The study was funded by Genzyme, a drug company that makes a drug called Thyrogen. The purpose of Thyrogen is to allow thyroid cancer patients to stop taking their regular thyroid hormone replacement drug, in preparation for a radioiodine scan, without going hypothyroid and experiencing hypothyroidism symptoms. So it's worth mentioning that the study was commissioned and funded by Genzyme as a way to help market Thyrogen to the thyroid cancer patient community, while justifying the need for the drug by linking hypothyroidism to impaired driving.

A Final Note: Patient "Non-Compliance?"

In his video, Dr. Ain said that in the Colorado Thyroid Prevalence Study, they found that almost a third of patients treated for hypothyroidism are "not compliant" with their therapies, making these patients a "profound public health menace" given the link between hypothyroidism and impaired driving.

While Dr. Ain's efforts to look at the hypothyroidism-driving link are appreciated, he has unfortunately misrepresented this particular study finding, and inappropriately placed blame on solely patients.

What the Colorado study actually found was that among patients taking thyroid medication, about 40% had TSH levels that fell outside the normal reference range for the TSH test.

In the medical world, to call patients "non compliant" means that the patient failed or refused to take a prescribed medication or treatments.

The Colorado Study did not explain why 40% of the patients being treated for hypothyroidism — which translates to millions of thyroid patients — fell outside the reference range. Certainly, some of these patients may be failing to take their medication, making them non-compliant. But it's also inaccurate to refer to the entire group as non compliant, when it's likely that this group also includes:

  • Patients whose doctors are underdosing/improperly treating their hypothyroidism
  • Patients whose doctors mistakenly feel that TSH levels levels between 5 and 10 reflect adequate treatment
  • Patients who do not properly absorb thyroid medication
  • Patients with pituitary dysfunctions whose thyroid treatment cannot be adequately monitored with TSH testing
  • Patients who have elevated heterophile antibodies falsely affecting TSH levels

Since we now know that undiagnosed, untreated or undertreated hypothyroidism makes some thyroid patients a danger behind the wheel, there's every reason to expand efforts to ensure proper thyroid diagnosis and treatment.

But let's also acknowledge that in the area of thyroid hormone replacement treatment, the fact that 40% of "treated" patients still fall outside the TSH reference range may be due just as much — if not more — to the doctors treating those patients, versus blaming patients for "non compliance."

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