Influence of cigarette smoking on thyroid hormone levels

Environmental Factors Affecting Thyroid-Stimulating Hormone and Thyroid Hormone Levels

Thyroid hormones are necessary for the normal functioning of physiological systems. Therefore, knowledge of any factor (whether genetic, environmental or intrinsic) that alters the levels of thyroid-stimulating hormone (TSH) and thyroid hormones is crucial. Genetic factors contribute up to 65% of interindividual variations in TSH and thyroid hormone levels, but many environmental factors can also affect thyroid function. This review discusses studies that have analyzed the impact of environmental factors on TSH and thyroid hormone levels in healthy adults. We included lifestyle factors (smoking, alcohol consumption, diet and exercise) and pollutants (chemicals and heavy metals). Many inconsistencies in the results have been observed between studies, making it difficult to draw a general conclusion about how a particular environmental factor influences TSH and thyroid hormone levels. However, lifestyle factors that showed the clearest association with TSH and thyroid hormones were smoking, body mass index (BMI) and iodine (micronutrient taken from the diet). Smoking mainly led to a decrease in TSH levels and an increase in triiodothyronine (T3) and thyroxine (T4) levels, while BMI levels were positively correlated with TSH and free T3 levels. Excess iodine led to an increase in TSH levels and a decrease in thyroid hormone levels. Among the pollutants analyzed, most studies observed a decrease in thyroid hormone levels after exposure to perchlorate. Future studies should continue to analyze the impact of environmental factors on thyroid function as they could contribute to understanding the complex background of gene-environment interactions underlying the pathology of thyroid diseases.

Thyroid redox imbalance in adult Wistar rats that were exposed to nicotine during breastfeeding

Maternal nicotine exposure causes several consequences in offspring phenotype, such as obesity and thyroid dysfunctions. Nicotine exposure can increase oxidative stress levels, which could lead to thyroid dysfunction. However, the mechanism by which nicotine exposure during breastfeeding leads to thyroid gland dysfunction remains elusive. We aimed to investigate the long-term effects of maternal nicotine exposure on redox homeostasis in thyroid gland, besides other essential steps for thyroid hormone synthesis in rats from both sexes. Lactating Wistar rats were implanted with osmotic minipumps releasing nicotine (NIC, 6 mg/kg/day) or saline (control) from postnatal day 2 to 16. Offspring were analyzed at 180-day-old. NIC males showed lower plasma TSH, T₃ and T₄ while NIC females had higher T₃ and T₄. In thyroid, NIC males had higher sodium-iodide symporter protein expression, whereas NIC females had higher thyroid-stimulating hormone receptor (TSHr) and thyroperoxidase (TPO) protein expression. TPO activity was lower in NIC males. Hydrogen peroxide generation was decreased in NIC males. Activities of superoxide dismutase, catalase and glutathione peroxidase were compromised in NIC animals from both sexes. 4-Hydroxynonenal was higher only in NIC females, while thiol was not affected in NIC animals from both sexes. NIC offspring also had altered expression of sex steroid receptors in thyroid gland. Both sexes showed similar thyroid morphology, with lower follicle and colloid size. Thyroid from female offspring exposed to nicotine during breastfeeding developed oxidative stress, while the male gland seemed to be protected from redox damage. Thyroid dysfunctions seem to be associated with redox imbalance in a sex-dependent manner.

The association between cigarette smoking and serum thyroid stimulating hormone, thyroid peroxidase antibodies and thyroglobulin antibodies levels in Chinese residents: A cross-sectional study in 10 cities

Objectives

Although several studies have shown that cigarette smoking is associated with thyroid stimulating hormone (TSH), thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb), the exact relationship between smoking and thyroid function is controversial. As little is known about the effects of smoking on TSH, TPOAb and TgAb in Chinese residents. This study aimed to evaluate the association between cigarette smoking and TSH, TPOAb and TgAb in ten-city residents of China.

Study design

This was a population-based cross-sectional study.

Methods

In this cross-sectional study, 15,181 subjects from ten major cities of China were investigated. Data regarding demographic characteristics, smoking status and consumption of iodine status were collected using in-person interviews based on a self-designed structured questionnaire. Serum concentrations of TSH, TPOAb and TgAb were measured using electrochemiluminescence immunoassays. Univariate analysis and multivariate linear stepwise regression analyses were used to analyze the data.

Results

The regular smokers had lower concentrations of TSH, TPOAb and TgAb than occasional smokers, former smokers and never smokers. Multivariate analysis demonstrated that regular smoking was associated with the decreased concentrations of TSH (β = -0.178), TPOAb (β = -0.287) and TGAb (β = -0.453) after adjusting other factors. Furthermore, daily smoking number was significantly associated with the decreased level of TSH (β = -0.045) and TPOAb(β = -0.080), and smoking duration was associated with the decreased TSH level (β = -0.030).

Conclusions

Our findings suggest that cigarette smoking is related to a significant decline in the concentrations of TSH, TPOAb and TgAb. In addition, daily smoking number and long-term smoking decrease serum TSH and TPOAb levels. Cigarette smoking plays a significant role in the development of thyroid dysfunction.

Pro inflammatory interleukins and thyroid function in Naswar (dipping tobacco) users: a case control study

BACKGROUND

Naswar is a type of finely ground, moistened smokeless dipping tobacco product being commonly used in Pakistan. Although, nicotine is the most important psychoactive agent present in Naswar, it also exerts immunosuppressive effects and could alter the levels of cytokines. Additionally, the effects of Naswar consumption on thyroid hormones are not known.

METHODS

Eighty healthy males aged 16-43 years were selected for the study and were divided into a control group comprising 31 healthy subjects with no history of tobacco use in any form, with age matched test group comprising 49 exclusive Naswar users who were consuming Naswar for at least 1 year. Estimation of serum interleukin (IL)-1β, IL-6, free thyroxine (FT4), free triiodothyronine (FT3) and thyroid stimulating hormone (TSH) was carried out. The data was analyzed by statistical programme (SPSS) using student's independent samples t-test. One way Anova followed by post hoc Tukey test was applied to assess parameters in Naswar users grouped according to duration of Naswar usage. Pearson's correlation coefficient was applied to assess correlations between parameters.

RESULTS

IL-1β was found to be significantly lowered in Naswar users compared to the control group whereas serum FT3 and FT4 levels in Naswar users were significantly raised compared to the control group. However, no differences in the levels of serum IL-6 and TSH between Naswar users and the control group were found. Also, serum FT3 and FT4 were consistently raised whereas IL-1β was lowered in Naswar users irrespective of duration of Naswar consumption. IL-1β was negatively correlated with FT3 in Naswar users.

CONCLUSION

The findings suggest that Naswar users might be in an immune suppressive state as evident by the lowered levels of interleukin 1β. Additionally, alterations in the levels of thyroid hormones signify the impact of Naswar consumption on thyroid function.

Effects of maternal nicotine exposure on thyroid hormone metabolism and function in adult rat progeny

Postnatal nicotine exposure leads to obesity and hypothyroidism in adulthood. We studied the effects of maternal nicotine exposure during lactation on thyroid hormone (TH) metabolism and function in adult offspring. Lactating rats received implants of osmotic minipumps releasing nicotine (NIC, 6 mg/kg per day s.c.) or saline (control) from postnatal days 2 to 16. Offspring were killed at 180 days. We measured types 1 and 2 deiodinase activity and mRNA, mitochondrial α-glycerol-3-phosphate dehydrogenase (mGPD) activity, TH receptor (TR), uncoupling protein 1 (UCP1), hypothalamic TRH, pituitary TSH, and in vitro TRH-stimulated TSH secretion. Expression of deiodinase mRNAs followed the same profile as that of the enzymatic activity. NIC exposure caused lower 5'-D1 and mGPD activities; lower TRβ1 content in liver as well as lower 5'-D1 activity in muscle; and higher 5'-D2 activity in brown adipose tissue (BAT), heart, and testis, which are in accordance with hypothyroidism. Although deiodinase activities were not changed in the hypothalamus, pituitary, and thyroid of NIC offspring, UCP1 expression was lower in BAT. Levels of both TRH and TSH were lower in offspring exposed to NIC, which presented higher basal in vitro TSH secretion, which was not increased in response to TRH. Thus, the hypothyroidism in NIC offspring at adulthood was caused, in part, by in vivo TRH-TSH suppression and lower sensitivity to TRH. Despite the hypothyroid status of peripheral tissues, these animals seem to develop an adaptive mechanism to preserve thyroxine to triiodothyronine conversion in central tissues.

Thyroid cancer risk and smoking status: a meta-analysis

PURPOSE

Previous researchers have reported an inverse association between cigarette smoking and thyroid cancer risk. To summarize the role of smoking in relation to thyroid cancer occurrence, we conducted a meta-analysis.

METHODS

We performed a meta-analysis of 31 eligible studies to summarize the data describing the association between thyroid cancer occurrence and smoking. The case-control studies consisted of 6,260 thyroid cancer cases and 32,935 controls. Cohort studies contained 2,715 thyroid cancer patients that participated from recruitment to follow-up. Q-statistic and I (2) statistic were calculated to examine heterogeneity. Summary relative risks (RRs) and 95 % confidence intervals (95 % CIs) were calculated using a random effects model. Potential sources of heterogeneity were investigated via subgroup and sensitivity analyses, and publication biases were estimated.

RESULTS

Thyroid cancer risk was reduced in persons who had ever-smoked (RR = 0.79; 95 % CI 0.70-0.88) compared with never-smokers. However, strong evidence of heterogeneity was found among the investigated studies; therefore, subgroup analyses were conducted according to study type, smoking status, study location, source of controls, sex, and histological type of thyroid cancer. When the data were stratified by smoking status, an inverse association was observed only among current smokers (RR = 0.74; 95 % CI 0.64-0.86), not former smokers (RR = 1.01; 95 % CI 0.92-1.10). An inverse association was observed only in case-control studies (RR = 0.75; 95 % CI 0.66-0.85).

CONCLUSIONS

This meta-analysis of geographically diverse epidemiological data suggests that smoking, particularly current smoking, may influence susceptibility to thyroid cancer. Further well-designed studies with larger sample sizes should be conducted.

Effect of maternal nicotine/thiocyanate exposure during gestational period upon pituitary, thyroid and parathyroid function/morphology of 1-month-old rat offspring

INTRODUCTION

Impact of in utero exposure to nicotine, on the structure of the thyroid-pituitary axis and the parathyroid glands have been examined in 1-month-old rats and compared with that of thiocyanate.

MATERIALS AND METHODS

Three pregnant female groups were used; control, nicotine and thiocyanate. Treatment started from gestation day (4-20) and the specimens were harvested from the male offspring of all groups at the age of 1 month and processed for light, electronmicroscopic and immunohistochemical examination. Total triiodothyronine (tT3), total thyroxine (tT4) and total thyrotropin (TSH) were quantitatively determined in serum.

RESULTS

Both nicotine and thiocyanate activated the thyroid follicular cells, with an increase in height (about 30 %) and a negative feedback on the pituitary thyrotrophs which revealed a reduction in the number of cytoplasmic secretory granules, particularly the thiocyanate group. However, in thiocyanate group there was signs of impaired secretory activity of the thyroid gland. The arbitrary area of parathyroid chief cells, increased (about 45 %) particularly in nicotine group, with signs of reduced activity and a positive feedback on the parafollicular cells which revealed hypertrophy, proliferation (25 %) and increased intensity of positive immunohistochemical reaction for calcitonin.

CONCLUSION

Nicotine impaired chief parathyroid cells activity and consequently activated parafollicular cells. Thiocyanate reduced pituitary thyrotrophs activity, whereas both nicotine and thiocyanate increased thyroid follicular cells activity. This impact of in utero exposure persisted for 1-month postnatal.

Lack of mutational events of RAS genes in sporadic thyroid cancer but high risk associated with HRAS T81C single nucleotide polymorphism (case-control study)

High incidence of thyroid cancer worldwide indicates the importance of studying genetic alterations that lead to its carcinogenesis. Specific acquired RAS mutations have been found to predominate in different cancers, and HRAS T81C polymorphism has been determined to contribute the risk of various cancers, including thyroid cancer. We screened the exons 1 and 2 of RAS genes (HRAS, KRAS, and NRAS) in 60 consecutive thyroid tissue (tumor and adjacent normal) samples, and a case-control study was also conducted for HRAS T81C polymorphism in HRAS codon 27 using the polymerase chain reaction-restriction fragment length polymorphism to test the genotype distribution of 140 thyroid cancer patients in comparison with 170 cancer-free controls from a Kashmiri population. No mutation was found in any of the thyroid tumor tissue samples, but we frequently detected polymorphism at nucleotide 81 (T > C) in exon 1 of HRAS gene. In HRAS T81C SNP, frequencies of TT, TC, and CC genotypes among cases were 41.4, 38.6, and 20.0 %, while in controls genotype frequencies were 84.1, 11.7, and 4.2 %, respectively. A significant difference was observed in variant allele frequencies (TC + CC) between the cases and controls (58.6 vs. 16 %) with odds ratio = 7.4; confidence interval (CI) = 4.3-12.7 (P < 0.05). Interestingly, combined TC and CC genotype abundantly presented in follicular thyroid tumor (P < 0.05). Moreover, a significant association of the variant allele (TC + CC) was found with nonsmokers (P < 0.05). This study shows that although thyroid cancer is highly prevalent in this region, the mutational events for RAS genes do not seem to be involved. Contrary to this HRAS T81C SNP of HRAS gene moderately increases thyroid cancer risk with rare allele as a predictive marker for follicular tumors.

Smoking and early pregnancy thyroid hormone and anti-thyroid antibody levels in euthyroid mothers of the Northern Finland Birth Cohort 1986

BACKGROUND

Smokers in the general population have lower thyrotropin (TSH) and higher free triiodothyronine (fT3) and free thyroxine (fT4) concentrations, but the results in pregnant population vary from no effect to a decrease in TSH and fT4 concentrations and an increase in fT3 levels. Our objective was to further evaluate the question of whether there is an association between smoking, before and during pregnancy, with maternal thyroid function during pregnancy and with the risk for subsequent hypothyroidism.

METHODS

Our study population was a prospective population-based cohort (N=9362), the Northern Finland Birth Cohort 1986, with extensive data throughout gestation. The mothers underwent serum sampling in early pregnancy. The samples were assayed for TSH, fT3, fT4, thyroid-peroxidase antibodies (TPO-Ab), and thyroglobulin antibodies (TG-Abs) (n=5805). Mothers with thyroid dysfunction diagnosed before or during pregnancy were excluded, leaving 4837 euthyroid mothers. The smoking status of mothers and fathers were requested by questionnaires during pregnancy. Subsequent maternal morbidity relating to hypothyroidism 20 years after the index pregnancy was evaluated using national registers.

RESULTS

Euthyroid mothers who smoked before, or continued smoking during first trimester of pregnancy, had higher serum fT3 (p<0.001) and lower fT4 (p=0.023) concentrations than nonsmokers. Smoking in the second trimester was associated with higher fT3 (p<0.001) concentrations, but no difference in fT4 concentrations compared with nonsmokers. TG-Abs were less common among smoking than nonsmoking mothers (2.5% vs. 4.7%, p<0.001), but the prevalence of TPO-Ab was similar. Paternal smoking had no independent effect on maternal early pregnancy thyroid hormone or antibody concentrations. The risk of subsequent maternal hypothyroidism after follow-up of 20 years was similar among prepregnancy smokers and nonsmokers.

CONCLUSIONS

In euthyroid women, smoking during pregnancy was associated with higher fT3 levels and lower fT4 levels; possibly reflecting smoking-induced changes in peripheral metabolism of thyroid hormones. No differences were found in TSH concentrations between smokers and nonsmokers. Our results differ from those of the general population, which usually have shown smoking-induced thyroidal stimulation. This is possibly due to pregnancy-induced changes in thyroid function. Decreases in fT4 levels among smokers might predispose to hypothyroidism or hypothyroxinemia during pregnancy. Despite these changes in thyroid function, smoking did not increase the woman's risk of subsequent hypothyroidism.

Neonatal nicotine exposure alters leptin signaling in the hypothalamus-pituitary-thyroid axis in the late postnatal period and adulthood in rats

AIMS

Postnatal nicotine exposure causes precocious primary hypothyroidism and programs for overweight, hyperleptinemia and secondary hypothyroidism in adulthood. As leptin and thyroid hormones share the ability to increase energy expenditure, we studied the effects of maternal nicotine exposure during lactation on the leptin signaling in the hypothalamus-pituitary-thyroid axis of suckling and adult offspring.

MAIN METHODS

Two days after delivery, osmotic minipumps were implanted in lactating rats, and nicotine (NIC, 6 mg/kg/day s.c.) or saline (C) was administered for 14days. Offspring were killed at 15 and 180 days-old. Proteins belonging to leptin signaling were analyzed by Western blot. Significant differences had p<0.05.

KEY FINDINGS

In the hypothalamus, NIC offspring showed higher OB-R and pSTAT-3 content (+58%,+1.34x) at 15 days, and lower OB-R, JAK-2 and pSTAT-3 (-61%, -42%, -56%) at 180 days. In the pituitary gland, NIC offspring showed lower JAK-2 content (-52%) at 15 days, but no differences in adulthood. In the thyroid gland, the NIC group presented lower OB-R, JAK-2 and STAT-3 (-44%, -50%, -47%) and higher pSTAT-3 expression (+80%) at 15 days. At 180 days-old, NIC offspring presented higher thyroid OB-R (+1.54x) and lower pSTAT-3 content (-34%).

SIGNIFICANCE

Neonatal primary hypothyroidism induced by maternal nicotine exposure during lactation may be partially explained by decreased leptin signaling in the thyroid, though the early stimulation of the central leptin pathway did not prevent the thyroid dysfunction. Long-term effects of postnatal nicotine exposure on leptin signaling in the hypothalamus and thyroid appear to involve central and peripheral leptin resistance in adulthood.

Cigarette smoking during pregnancy is associated with alterations in maternal and fetal thyroid function

CONTEXT

Studies in the general population have shown lower serum TSH levels in smokers as compared with nonsmokers.

AIM

Our aim was to examine whether smoking is associated with changes in thyroid function of pregnant women and their fetus.

SUBJECTS AND METHODS

We examined the relationship between smoking and thyroid function (serum TSH, free T4, and free T3) in two independent cohorts of pregnant women without a history of thyroid disorder or an overt biochemical thyroid dysfunction: 1) first-trimester cohort (median gestation 9 wk) (n = 1428) and 2) third-trimester cohort (gestation 28 wk) (n = 927). We also analyzed the relationship between maternal smoking and thyroid hormone levels in cord serum of 618 full-term babies born to the women in the third-trimester cohort.

RESULTS

In smokers compared with nonsmokers, median serum TSH was lower (first-trimester cohort: 1.02 vs. 1.17 mIU/liter, P = 0.001; third-trimester cohort: 1.72 vs. 1.90 mIU/liter, P = 0.037), and median serum FT3 was higher (first-trimester cohort: 5.1 vs. 4.9 pmol/liter, P < 0.0001; third-trimester cohort: 4.4 vs. 4.1 pmol/liter, P < 0.0001). In both cohorts, serum FT4 in smokers and nonsmokers were similar. The prevalence of anti-thyroperoxidase antibodies was also similar in smokers and nonsmokers in both cohorts. Cord serum TSH of babies born to smokers was lower than of those born to nonsmokers (6.7 vs. 8.1 mIU/liter, P = 0.009).

CONCLUSIONS

Cigarette smoking is associated with changes in maternal thyroid function throughout the pregnancy and in fetal thyroid function as measured in cord blood samples.

Cigarette smoking and iodine as hypothyroxinemic stressors in U.S. women of childbearing age: a NHANES III analysis

BACKGROUND

Exposure to environmental thiocyanate through smoking has been suggested to lead to hypothyroxinemia, which potentially impairs brain development in the fetuses of affected women, though studies are conflicting. It was hypothesized that iodine status might modulate the effects of thiocyanate exposure on the prevalence of hypothyroxinemia in women of childbearing age.

DESIGN

The study population comprised 6967 women (age range: 15- 44 years) from the National Health and Nutrition Examination Survey (NHANES) III database. Smoking status was stratified into nonsmokers and those who smoked 1-10, 11-20, 21-30, and 31+ cigarettes a day. Iodine status was stratified based on urinary iodine excretion as <50, 50-99, 100-199, 200-299, and 300+ microug/L. Hypothyroxinemia was defined as the lower fifth percentile of total thyroxine levels among nonsmokers, adjusted for age and race/ethnicity. Univariate, multivariate, and regression analyses were conducted to evaluate the impact of smoking and urinary iodine excretion on the prevalence of hypothyroxinemia.

RESULTS

Increasing levels of cigarette smoking are associated with increasing prevalence of hypothyroxinemia [chi(2)(4) = 14.15, p = 0.007]. When analyzed by urinary iodine level, the hypothyroxinemic effect of smoking was limited to the highest two urinary iodine strata [chi(2)(4) = 41.48, p < 0.001; and chi(2)(4) = 40.62, p < 0.001]. A significant interaction effect between smoking and urinary iodine was noted, underscoring the relationship between high levels of urinary iodine excretion and smoking with respect to hypothyroxinemia.

CONCLUSIONS

Heavy smoking was associated with a higher prevalence of hypothyroxinemia. The impact of thiocyanate exposure from smoking on the prevalence of hypothyroxinemia is limited to those women of childbearing age with the highest urinary iodine excretion. Iodine supplementation should be cautiously considered in women of childbearing age who are smokers.

A case-control study in Hiroshima and Nagasaki examining non-radiation risk factors for thyroid cancer

BACKGROUND

Because little is known about the etiology of thyroid cancer in Japan, we conducted a case-control study of thyroid cancer and lifestyle and other risk factors. The present report focuses on medical history, family history, smoking and alcohol drinking, and their interactions with radiation exposure.

METHODS

Thyroid cancer cases reported to the Hiroshima and Nagasaki tumor registries during 1970-1986 were histologically reviewed by pathologists. For each of 362 cases with papillary or follicular adenocarcinoma diagnosed at <75 years of age, one control without cancer matched on city, sex, year of birth, and atomic-bomb radiation exposure was selected from the Life Span Study cohort or the offspring cohort. The cohort subjects were residents of Hiroshima and Nagasaki with or without atomic-bomb radiation exposure. Information on risk factors was obtained through a pre-structured interview carried out in 1986-1988.

RESULTS

Analysis using conditional logistic regression showed history of goiter or thyroid nodule and family history of cancer to be significantly associated with an increased odds ratio for thyroid cancer. Smoking and alcohol drinking were significantly and independently associated with a reduced odds ratio. Interaction between smoking and alcohol drinking was not evident based on either an additive model or a multiplicative model. Radiation exposure did not significantly modify the associations between these factors and thyroid cancer risk.

CONCLUSION

History of goiter/nodule and family history of cancer were risk factors for thyroid cancer. Smoking and alcohol drinking were independently associated with reduced risk. Self-reported retrospective information presents some limitations in interpretation of the data.

Smoking and hormones in health and endocrine disorders

Smoking has multiple effects on hormone secretion, some of which are associated with important clinical implications. These effects are mainly mediated by the pharmacological action of nicotine and also by toxins such as thiocyanate. Smoking affects pituitary, thyroid, adrenal, testicular and ovarian function, calcium metabolism and the action of insulin. The major salient clinical effects are the increased risk and severity of Graves' hyperthyroidism and opthalmopathy, osteoporosis and reduced fertility. Smoking also contributes to the development of insulin resistance and hence type 2 diabetes mellitus. An important concern is also the effect of smoking on the foetus and young children. Passive transfer of thiocyanate can cause disturbance of thyroid size and function. Furthermore, maternal smoking causes increased catecholamine production, which may contribute to under perfusion of the foetoplacental unit.

Thiocyanate overload and thyroid disease

Thiocyanate [SCN-] is a complex anion which is a potent inhibitor of iodide transport. It is the detoxification product of cyanide and can easily be measured in body fluids. Consumption of naturally occurring goitrogens, certain environmental toxins and cigarette smoke can significantly increase SCN- concentrations to levels potentially capable of affecting the thyroid gland. Goiter endemics were reported to develop when the critical urinary iodine/ SCN- ratio decreases below 3 microg iodine per mg SCN-. Iodine supplementation completely reverses the goitrogenic influence of SCN-. SCN- is also generated from cigarette smoking as a detoxifying product of cyanide. During the past two decades many reports dealt with the possible effects of cigarette smoking on thyroid hormone synthesis, thyroid gland size and thyroid autoimmunity including infiltrative ophtalmopathy of Graves' disease. In this mini-review, issues regarding thiocyanate overload and thyroid disease will be summarized.

Risk factors for goiter and thyroid nodules

The occurrence of thyroid diseases is determined by interplay between genetic and environmental factors. The major environmental factor that determines goiter prevalence is iodine status, but other environmental factors influencing entire populations have been identified such as goitrogens in food and drinking water. Less focus has been on individual environmental factors and the interplay between factors. The goiter prevalence is higher in certain groups in the population. The variation in goiter prevalence between the genders is well known with a higher occurrence among women. The association with age is probably dependent on iodine status, because it seems that the zenith of goiter prevalence appears earlier in life the more severe iodine deficiency the population is exposed to. The association with individual risk factors has been investigated in some studies, especially the association with tobacco smoking. In iodine-deficient areas, a strong association between tobacco smoking and goiter prevalence is found, whereas the association is less pronounced in iodine-replete areas. This was predictable from experimental studies showing thiocyanate to be the mediator of the goitrogenic effect of tobacco smoke acting as a competitive inhibitor of iodine uptake. The association with alcohol intake has only been investigated in few studies, but a low occurrence of goiter among alcohol consumers has been found. The mechanism of this association is not known. Increased goiter prevalence during pregnancy has been reported, and recently a long-term goitrogenic effect of pregnancies has also been shown. As demonstrated for tobacco smoking, this association is dependent on iodine status, because the association has only been found in areas with a suboptimal iodine intake. This indicates pregnancy-induced goiter to be the result of exacerbation of existing iodine deficiency. Recently, the use of oral contraceptives has been shown to be associated with a markedly reduced prevalence of goiter, although experimental studies have previously shown proliferative effects of estrogens on thyrocytes. Some implications for prevention of thyroid disease could be suggested. Discussion of smoking habits should be included in a consultation for goiter with a motivation to quit smoking. Iodine deficiency has particularly strong goitrogenic effects during pregnancy and for the sake of the mother as well as the fetus, sufficient iodine supply should be ensured to all pregnant women. The difference in age maximum in goiter prevalence suggests that monitoring of iodine deficiency disorders should ideally include a spectrum of age groups.

Cigarette smoking and the risk of thyroid cancer

Thyroid cancer, whose aetiology is largely uncertain, has been negatively associated with cigarette smoking in a number of studies, possibly consistent with the greater occurrence of the disease in women than in men. This association was explored in the context of a Canadian case-control study of thyroid cancer. Newly diagnosed cases were identified primarily through provincial cancer registries in Canada and controls were identified from the general population. Data were collected through mailed questionnaires, yielding 1224 cases and 2659 controls. Reduced risk was observed for ever/never cigarette smoking (risk ratio estimate (RR) of 0.71 (95% confidence interval (CI)=0.60-0.83) for females and 0.77 (95% CI=0.58-1.02)) for males. Dose-response effects were observed with duration, quantity smoked and pack-years of exposure, although there was no decreasing protective effect with the age started smoking or years since stopped smoking. There is evidence of reduced risk for all histological subgroups. The protective effect of smoking may be due to a number of different mechanisms, including an effect on thyroid stimulating hormone and on oestrogen metabolism.

Magnesium modulates contractile responses of rat aorta to thiocyanate: A possible relationship to smoking-induced atherosclerosis

Thiocyanate anions (SCN-) as the end products of tobacco smoke and found in the blood of cigarette smokers have been implicated in atherogenesis and heart diseases. Magnesium deficiency has also been implicated in the etiology of atherogenesis. The contractile responses of rat aorta to SCN- and the modulation of extracellular magnesium ions ([Mg2+]o) on the effect of SCN- were, therefore, studied in isolated rat aortic rings. SCN- exposure at a range of concentrations (from 10(-5) to 5 x 10(-2) M) induces contractile responses of isolated rat aortic rings with and without endothelium in a concentration-dependent manner. Significant differences in responsiveness to SCN- were found in rat aortic ring segments with and without endothelial cells. Preincubation of these vessels with low [Mg2+]o markedly shifted the contractile concentration-effect curves to the left, and the contractile effects of SCN- in rat aortic rings were potentiated. In contrast to lowering [Mg2+]o, increasing [Mg2+]o to 2.4 mM was found to dramatically attenuate the contractile responses to SCN-. In the absence of extracellular Ca2+ ([Ca2+]o), SCN--induced contractions were, however, almost abolished after exposure to Mg2+-free medium. In order to investigate the mechanisms of [Mg2+]o modulation of SCN--induced contractile response of rat aorta, changes in intracellular Ca2+ ([Ca2+]i) were measured in cultured primary smooth muscle cells isolated from rat aorta. The resting level of [Ca2+]i in the rat aortic smooth muscle cells was 80.6 +/- 6.6 nM. Exposure of these cells to SCN- (5 x 10(-5) to 5 x 10(-3) M) produced rises in [Ca2+]i in a concentration-dependent manner. Preincubation of these cells with low [Mg2+]o (0 or 0.3 mM, the lowest physiological range) for 24 h significantly potentiated increments in [Ca2+]i induced by SCN-. These rises in [Ca2+]i induced by SCN- were completely inhibited by pretreating the cells with 2.4 mM [Mg2+]o for 24 h. These results support a hypothesis whereby cigarette smoking or exposure to smoking can induce cardiovascular diseases, at least partly, probably by causing spasm and thickening of arterial blood vessels as a consequence of large rises in [Ca2+]i in vascular smooth muscle cells. The chronic presence of or exposure to both thiocyanate and low Mg2+ in the blood of smokers can result in rapid flux of Ca2+ into vascular smooth muscle cells, thus accelerating or initiating atherosclerotic processes in smokers.

The effect of three products of cigarette smoke (cyanide, thiocyanate and nicotine) on the concentration-response curves of 5-hydroxytryptamine, norepinephrine and epinephrine in the isolated human umbilical veins and arteries

The effect of cyanide (10(-7) M), thiocyanate (10(-4) M) and nicotine (10(-7) M) on the concentration-response curves of 5-hydroxytryptamine, norepinephrine and epinephrine were investigated in human isolated umbilical arteries and veins. Cyanide significantly affected the responses of arterial strips to 5-hydroxytryptamine, norepinephrine and epinephrine: It caused significant leftward shifts of the 5-hydroxytryptamine concentration-response curves and significantly potentiated the contractile effects of norepinephrine and epinephrine in the case of norepinephrine, and epinephrine concentration reached 10(-6) and 10(-7) M respectively in the bath medium. Cyanide did not show any significant effect on the concentration-response curves of 5-hydroxytryptamine, norepinephrine and epinephrine in veins. Nicotine interacted with the response of adrenergic agonists both in arteries and veins; in arteries it potentiates the contractile response of epinephrine; in veins, it inhibited the dilatory responses of norepinephrine and potentiated the contractile effect of high concentration of epinephrine (10(-6) M). Thiocyanate did not cause any difference on any cumulative concentration-response curves either on the vessels. However, none of these individual effects of cyanide and nicotine were observed when the cyanide, thiocyanate and nicotine were added in combination in the isolated organ bath medium.

Cigarette smoking and the thyroid

Relevant English language articles published from 1970 through 1993 regarding the possible influence of cigarette smoking on the thyroid were identified through a MEDLINE search and manual searches of identified articles. Thiocyanate in tobacco smoke influences the thyroid by a competitive inhibition of iodine uptake and organification in the gland. Also the stimulation of the sympathetic nervous system by cigarette smoke and benzpyrene, another constituent of tobacco, is thought to influence thyroid gland function. The thyroid hormones and TSH receptor autoantibodies are not affected by smoking, but serum TSH levels have been found to be slightly reduced. Smokers have a higher frequency of goiter and increased serum thyroglobulin levels, especially in iodine-deficient areas. Graves' ophthalmopathy is strongly associated with cigarette smoking; the more severe the eye disease the stronger the association. Graves' disease without ophthalmopathy is also associated with smoking, though this association is weaker. Thiocyanate level in cord blood equilibrates completely with the level in the mother, and a reverse correlation has been demonstrated between birth weight and thiocyanate level in cord blood. Cigarette smoking induces similar changes in thyroid function in the adult and the fetus. No separate study has elucidated the effects of cessation of smoking, but there seems to be longstanding effects induced by smoking, some probably irreversible.