Among patients with subclinical hypothyroidism,
those who smoke have higher serum thyrotropin
concentrations, a higher serum ratio of tri-iodothyronine
to free thyroxine, and higher serum cholesterol and lowdensity lipoprotein cholesterol concentrations than
those who do not smoke (6). In contrast, among
patients with overt hypothyroidism, smokers and nonsmokers have similar serum thyrotropin and thyroid
hormone concentrations, but more symptoms, signs
and biochemical and physiological changes of hypothyroidism (6). Thus smoking reduces thyroid secretion in
patients with subclinical hypothyroidism and exacerbates the peripheral effects of thyroid deficiency in those
patients and in patients with overt hypothyroidism.
Notwithstanding these results, in the same case-control
studies cited above smoking was not a risk factor for
chronic autoimmune thyroiditis (4, 5), although it was
associated with postpartum thyroiditis, a precursor of
chronic autoimmune thyroiditis, in one study (7).