Thyroid Hormone Levels and Cigarette Smoking in Baboons 1

Abstract

Abstract

Using a primate animal model, two studies were undertaken to examine the effects of cigarette smoking on thyroid hormone levels. In study 1, mean total triiodothyronine (total T₃) and mean total thyroxine (total T₄) levels were measured in two groups of baboons (Papio cynocephalus) who were taught to smoke cigarettes using operant conditioning techniques. The smokers were divided into established and naive smokers according to pack-years of exposure. A control group of never-smoker baboons was included for comparison. Blood sampling was done after long-term cigarette consumption and again 1 week after cigarette deprivation. In the naive smoker group, mean total T₃ concentrations were reduced below control group values (P < 0.05). After cigarette deprivation for 1 week, mean total T₃ values returned to normal. No significant differences in total T₄ levels were observed in either group. In study 2, we assessed some other indices of thyroid function. The same groups of baboons were divided into good and poor smokers by plasma cotinine and blood carboxyhemoglobin (% COHb) levels during 28 weeks of cigarette smoking activity. Immediate fluctuations and reductions in total T₃ levels were observed that were not accompanied by reductions in total T₄. The animals were then cigarette deprived for 1 week and blood samples were obtained every other day during this period. Significant increases in total T₃ concentrations were observed in poor smokers immediately after cessation. Both groups also exhibited significant reductions (P < 0.05) in T₃ uptake and free T₄ index (FT₄I) when compared to control group values. These data suggest that poor smokers are more susceptible to thyroid hormone level shifts than more established smokers, since the established smokers become habituated to the compounds contained in cigarette smoke through repeated exposure.