March 21, 2000 (Los Angeles) -- Need yet another reason to quit smoking? A new study from Denmark shows a strong link between cigarette smoking and thyroid disease. The authors say their analysis of 132 pairs of twins in which one member developed thyroid disease and the other did not provides "irrefutable" evidence of an association between smoking and thyroid damage.
"This represents a new way of looking at the possible relationship between smoking and thyroid disease," says endocrinologist Robert D. Utiger, MD, who reviewed the study for WebMD. "The twin approach is a novel one."
Study author Thomas Heiberg Brix, MD, and colleagues at Odense University and Odense University Hospital in Denmark studied 132 pairs of identical and fraternal twins born from 1953 to 1972. The researchers found what they call "a significant association" between smoking and thyroid disease. The amount of tobacco consumption also was significant: In the 51 pairs of twins in which both members smoked, tobacco use had been heavier in the twin who developed thyroid disease.
Smoking was found to be associated with both overactivity and underactivity of the gland. Symptoms of an overactive thyroid, which doctors call hyperthyroidism, include heart palpitations, insomnia, fatigue, hair loss, and a feeling of being hot most of the time. Symptoms of an underactive thyroid, or hypothyroidism, include fatigue, weakness, muscle cramps, constipation, and feeling cold most of the time.
In this study, smoking was most strongly associated with forms of thyroid disease in which the patient's immune system attacks the gland as if it were an invading organism. Utiger believes this might occur because smoking facilitates the production of defense mechanisms against one's own body. Graves' disease, one of the conditions that results from this phenomenon, is characterized not just by an overactive thyroid but also by swelling and inflammation of the muscles and tissues around and behind the eye, which cause the eyes to bulge, Utiger says. Patients with this condition may see double because their eye movements are no longer coordinated, and their eyes may tear excessively.
Earlier studies have demonstrated a relationship between smoking and all kinds of thyroid disease, says Utiger, a clinical professor of medicine at Harvard Medical School. "It is one of the mysteries of the topic, how smoking can be associated with all of these conditions," he tells WebMD. "In my opinion, there is no plausible explanation for why smoking is associated with an overactive thyroid on the one hand and an underactive thyroid on the other hand. It could be that smoking simply damages the thyroid in general, and that damage expresses itself in different ways in different individuals." Some evidence suggests that patients with eye problems improve more quickly when they stop smoking.
Cigarette smoke affects so many different organs because it contains "a whole carload of chemicals," Utiger says. "Nicotine, cyanide, hydrocarbons -- there are a whole host of poisons, with effects on a variety of tissues, rather than just one unifying poison." He believes the most important message of this study is that "these data provide further evidence that smoking is associated with several different kinds of thyroid disease."
- A new study shows a strong association between cigarette smoking and different types of thyroid disease.
- The cumulative effects of cigarette smoking were important, as those who smoked more were more likely to develop thyroid disease.
- Researchers do not know how smoking can cause either overactivity or underactivity of the thyroid gland.