Impaired action of thyroid hormone associated with smoking in women with hypothyroidism

Smoking may reduce both the production and physiologic action of thyroid hormone. Thyrotropin from the pituitary gland stimulates the thyroid gland to produce thyroxine, which in turn is converted to active thyroid hormone, known as triiodothyronine. Researchers measured the levels of all of these hormones in 138 women with normal thyroid function, 84 with asymptomatic hypothyroidism, and 51 with symptomatic hypothyroidism. Among the normal women and those with symptomatic hypothyroidism, hormone levels were the same in non-smokers and smokers. However, among the women with asymptomatic hypothyroidism, smokers had higher cholesterol and thyrotropin levels and higher ratios of triiodothyronine to thyroxine. These comparisons indicate that smoking harms the thyroid gland and inhibits the action of thyroid hormone on cells. These effects of smoking may not be apparent in women with advanced hypothyroidism because their thyroid gland is already damaged.