January 15, 2015
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Severe hypothyroidism can lead to cognitive, motor driving impairments

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Empirical evidence supports a recommendation that activities requiring rapid responses, including operating motor vehicles, should be avoided during severe periods of hypothyroidism, according to research published in Thyroid.

“Severe hypothyroidism can cause sufficient neurological impairment to constitute a special hazard for patients who participate in potentially dangerous activities, such as driving, that require quick and appropriate neurological responsiveness,” Kenneth B. Ain, MD, of the University of Kentucky College of Medicine, Lexington, told Endocrine Today.

Kenneth B. Ain

These findings carry broader implications regarding functional impairments and risk to public health, according to researchers.

Ain, along with Charles D. Smith, MD, and colleagues analyzed 32 outpatients with thyroid cancer to determine the specific neurological and psychological deficits experienced with hypothyroidism and gauge their magnitude. All patient had undergone thyroid hormone withdrawal preparation for radioiodine scanning.

The researchers obtained repeated measurements in euthyroid, hypothyroid and euthyroid hormone replaced states; in the longitudinal ‘‘A-B-A’’ study design, patients were tested at three visits in the same sequence.

Clinical status and cognitive performance data were gathered using standard instruments including ThyDQoL and ThySRQ measures, National Adult Reading Test, Boston Naming Test, Mini-Mental State Exam, Wechsler Adult Intelligence Test-Revised, Letter Fluency FAS and Beck Depression Inventory.

An automated assessment panel was used to measure fine-motor function, and a commercial driving simulator to measure driving performance.

In severe hypothyroidism (median thyrotropin, 83.2 mIU/L), patients demonstrated slowed fine-motor performance of hands and reaction times in emergency braking tests; subjective slowing also was reported on structured clinical scales.

Although depression was present and typified by somatic and changing mood, patients did not report guilt and low self-esteem seen with other forms of depression.

Cognitive impairment was demonstrated by declines on speeded executive tests; however, episodic memory performance improved with time, irrespective of thyroid hormone status.

Braking times in hypothyroidism increased by 8.5%, which the researchers highlighted as equivalent to reports of effects from a blood alcohol level of 0.082 g/100 mL, above the US legal driving limit.

“When physicians encounter severely hypothyroid patients, they are ethically and legally required to warn them against operating motor vehicles or engaging in potentially dangerous activities that are made hazardous by their hypothyroid state,” Ain said. “Such hazards exist for those patients and for the public exposed to their compromised performance.”

The findings provide support for clinical recommendations for patients to emphasize caution when engaging in potentially dangerous activities during a severely hypothyroid state, according to researchers; upon restoration to euthyroid state, the stipulations could be confidently removed.

Determining the minimal time interval after treatment for restoration of normal driving performance was not part of the study design.

“Additional studies need to be done regarding the time-course of euthyroid recovery of neurological competence and the degrees of hypothyroidism associated with compromised neurological functioning,” Ain said. – by Allegra Tiver

For more information:

Kenneth B. Ain, MD, can be reached at the Thyroid Oncology Program, Room MN524, University of Kentucky Medical Center, 800 Rose St., Lexington, KY 40536-0298; email: kenneth.ain@uky.edu.

Disclosure: This work was funded by a grant from Genzyme, a Sanofi company.