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Reversible Cognitive, Motor and Driving Impairments in Severe Hypothyroidism

Thyroid. 2014 Nov 8. [Epub ahead of print]

Reversible Cognitive, Motor and Driving Impairments in Severe Hypothyroidism.

Smith CD1, Grondin R, LeMaster WB, Martin BJ, Gold BT, Ain K.

Author information

1University of Kentucky, Department of Neurology, Lexington, Kentucky, United States ;


Background: Hypothyroidism has been associated with cognitive and motor impairments that are likely to constitute hazards in the operation of motor vehicles and a public safety risk; however, there is a paucity of data that would provide an evidence basis for recommendations to hypothyroid patients. The purpose of this study was to determine the specific neurological and psychological deficits consequent to hypothyroidism and whether they are of sufficient magnitude to impede the safe operation of motor vehicles.

Methods: Repeated measurements were obtained in euthyroid, hypothyroid, and euthyroid hormone replaced states of thyroid cancer outpatients, at an academic medical center, who underwent thyroid hormone withdrawal preparation for radioiodine scanning. Study design used within-subject longitudinal "A-B- A" with each subject tested at 3 visits in the same sequence: euthyroid, hypothyroid, and euthyroid for a total of 32 subjects. Data on clinical status and cognitive performance were collected 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 inventory. Fine motor function was measured with an automated assessment panel, and driving performance on a commercial driving simulator.

Results: In severe hypothyroidism (median TSH 83.2 mIU/L), fine motor performance of hands and reaction times in emergency braking tests were slowed, as well as subjective slowing reported on structured clinical scales. Depression was present, typified by vegetative and mood alterations, but lacking reported guilt and lowered self-esteem seen in other types of depression. Cognitive impairment was characterized by declines on speeded executive tests. In contrast, episodic memory performance improved over time regardless of thyroid hormone status. Braking times increased in hypothyroidism by 8.5%, equivalent to reports of effects from a blood alcohol level of 0.082 g/100 mL (above the USA legal driving limit).

Conclusions: Transient profound hypothyroidism is characterized by reversible depression, decreased fine motor performance, slowed reaction times, and decreased processing speed. These data represent new empirical evidence that support the recommendation that complex activities requiring rapid responses, such as operating motor vehicles, should be avoided during hypothyroidism. This has broader implications regarding functional impairments and risk to public health. 



[PubMed - as supplied by publisher]


5 Replies

I find reversing quite difficult.....

1 like

Lol that was all I could do properly! Going forwards was my problem. I don't drive but cycle and I had to stop that when I was overtly hypo I just did not feel safe at all I was anxious and uncoordinated. I am fine now might even try driving lessons....


What about low grade hypothyroidism for a prolonged period of time? Like years? I guess that would be actionable. Best to not look too closely.

1 like

I am sure there is so truth in this as I pranged my car in the summer (first time in 35 years) when over medicated on t4 and feeling awful. However I think they have to be careful where the research takes them. You have slower reaction times as you get older, when you are sleep deprived after having a baby. Is the logical conclusion that nobody with hypothyroidism, old, just had a baby any other kind of illness allowed to drive??


In one sense, this is very distressing to me. In another sense, it explains a lot! Even more reason to keep the thyroid hormone levels healthy!


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