This is a re-post of something I first posted here some four years ago:
Looking for something else entirely (and nothing to do with thyroid at all!), I happened across a north American web site discussing various medical conditions and driving. Of course, me being me, couldn't stop myself. I downloaded the whole site as a PDF (option on left if you follow link below the text).
9.2. Thyroid Disease
Hyperthyroidism is the clinical expression of a group of disorders that produces elevated levels of free thyroxine and/or triiodothyronine (Gorroll, May, and Mulley, 1987).
Disorders include toxic goiter (Grave's disease), toxic multinodular goiter, and toxic uninodular goiter.
Although the prevalence of hyperthyroidism is not known precisely, community-based studies report prevalence rates of 1.9 percent in women and 0.16 percent in men (Gorroll, May, and Mulley, 1995). Approximately 15 percent of recognized cases occur in people older than 60 (Gorroll et al., 1995).
Clinical symptoms of hyperthyroidism include nervousness, tremor, muscle weakness, increased appetite, weight loss, and heat intolerance. In the elderly individual, symptoms may be atypical, with the patient presenting with apathy, weight loss, and cardiovascular dysfunction (unexplained atrial fibrillation). A number of therapeutic agents are available for the treatment of hyperthyroidism.
Of the thyroid disorders, hypothyroidism is the more common. The condition is most often caused by some disorder of the thyroid gland that causes decreased thyroid hormone production and secretion (Barnes, 1990).
Iodine deficiency is the most common cause worldwide of hypothyroidism. In regions where iodine intake is adequate, the most common causes are chronic autoimmune thyroiditis (Barnes, 1990).
Clinically, the patient with hypothyroidism presents with the following symptoms: fatigue, lethargy, sleepiness, dry skin, cognitive impairment, intolerance to cold, and weight gain. Fatigue, sleepiness, and cognitive impairment are the symptoms with the greatest relevance for driving. Once the diagnosis is established, treatment consists of thyroid hormone replacement therapy.
As with hyperthyroidism, hypothyroidism affects more women than men. The prevalence of hypothyroidism increases with age. According to Goroll et al. (1987), as much as five percent of the elderly population show evidence of hypothyroidism. Subclinical hypothyroidism, on the other hand, is estimated to affect between four percent and 14 percent of people older than 60, with more females than males affected.
Hypothyroidism and Driving Literature Review
As noted previously, cognitive impairments, sleepiness, and fatigue associated with hypothyroidism have direct relevance for driving. In terms of research literature, the effects of hypothyroidism on cognitive functioning have received the most attention. Cognitive deficits associated with hypothyroidism include impairments in general intelligence (Haggerty, Evans, and Pringe, 1986; Mennemeier, Garner, and Heillman, 1993), attention and concentration (Osterweill, Syndulko, Cohen, et al., 1992), memory (Haggerty et al., 1986; Mennemeier et al., 1993), perceptual and visual functioning (Mennemeier et al., 1993; Osterweill et al., 1992), and executive/frontal lobe functioning (Mennemeier et al., 1993). It is interesting to note that many of the cognitive deficits associated with hypothyroidism do not show consistent improvement following treatment with thyroid hormone replacement therapy. It may, therefore, be important to test for cognitive deficits in individuals with hypothyroidism once they have been stabilized on thyroid hormone replacement therapy.
[My emphasis above.]
Go to Section 9 Metabolic disorders.
And specifically the Canadian Medical Association guidelines are quoted in that document:
Patients with hyperthyroidism complicated by cardiac or neurologic symptoms should not drive any type of motor vehicle until the condition has been controlled.
Patients with symptomatic hypothyroidism should not drive any type of motor vehicle until the condition has been brought under satisfactory control.
Be careful out there...