Thyroid hormone concentrations in rats after chronic nicotine metabolite administration.

In an attempt to evaluate the observed relationship of chronic cigarette smoking and reduced thyroid hormone activity, the major urinary metabolites of nicotine were administered to rats for 78 weeks. The animals were divided equally into one control (n = 33) and three treatment groups. Treatment group 1 received 0.1% (w/v continine, group 2 received 0.02% pure trans-nicotine-N'-oxide, and group 3 received 0.02% of a trans/cis mixture (64/36%) of nicotine-N'-oxide. Plasma and urinary nicotine and cotinine concentrations were determined as well as a variety of thyroid hormone parameters. Pure trans-nicotine-N'-oxide was more extensively metabolized to its cotinine end product, relative to the diasteromeric N'-oxides, mixture. Serum triiodothyronine (T3) was markedly reduced in animals receiving nicotine-N'-oxides, but was not different in the cotinine treatment group when compared to control values. A reduction in serum thyroxine (T4) values was noted only among those rats receiving the pure trans-nicotine-N'-oxide. The T3/T4 ratio, free T3 index, T3 uptake, and rT3 were altered in animals receiving nicotine-N'-oxides. These findings indicate that specific nicotine metabolites alter thyroid hormone concentrations after chronic low-dose administration and possibly do so through back conversion to the parent compound, nicotine.