Influence of Cigarette Smoking on Thyroid Gland

Introduction

Exposure to tobacco smoke is reported to have variable effects on thyroid function. Several studies used to assess smoking status have shown that smokers have higher levels of thyroid hormones than nonsmokers. Some recent studies revealed that smokers had lower levels of thyroid peroxidase antibodies than nonsmokers, which proves that nonsmokers have a decreased prevalence of autoimmune thyroid disorders.

The self-reported effects of tobacco smoking on thyroid function are used to evaluate smoking and were categorized into active or passive and nonsmoker groups, as well as former or current or never-smoker groups. Potential bias in information could influence the interpretation and outcome of the results. Therefore, the relationship between thyroid hormone and tobacco smoking should be assessed with validating tools for smoking, such as cotinine levels. Urinary cotinine levels evaluate smoking and analyze the relationship between cigarette smoking and thyroid hormones.

How Cigarette Smoking and Thyroid Function Are Related?

Cigarette smoking may exert multiple effects on the thyroid gland. Smoking induces changes in thyroid function tests, like a decrease in TSH and an increase in thyroid hormones. These alterations were usually mild. In addition, tobacco smoking may also play an important role in thyroid autoimmunity. Some studies have confirmed the significant influence of smoking on Grave's hyperthyroidism and Grave orbitopathy (swelling or bulging around the areas of the eyes). Smoking may increase the risk of development, reduce the effectiveness of treatment, and induce relapse.

Eventually, cigarette smoking increases the incidence of multinodular goiter, especially in those iodine-deficient areas. Studies have examined cigarette smoking with thyroid cancer. Smoking considerably reduces the risk of developing differentiated thyroid cancer. Both active and passive smoking during pregnancy may modify maternal and fetal thyroid function. Influence of cigarette smoking on the thyroid gland, including hormonal changes, autoimmunity, and selected diseases. Furthermore, studies are required to explain the effects of smoking on thyroid pathophysiology.

How Is Smoking Related to Thyroid Function?

Tobacco smoking contains harmful substances that affect the functioning of the thyroid gland as well as the thyroid gland tissues. For example, cyanide, a major component in tobacco, is converted into thiocyanate (harmful chemical) when smoked. Thiocyanate is known to interfere with thyroid function. In three ways, they are listed below:

• It inhibits the iodine absorption into the cells of the thyroid gland and thereby reduces the release of the thyroid hormones triiodothyronine (T3) and thyroxine (T4).

• Directly inhibits the production of hormones by blocking the synthesis of these hormones in the thyroid gland.

• Increases the iodine excretion from the kidneys, increasing the risk of thyroid gland inflammation and symptoms such as fever, nausea, and stomach pain.

The current smoking may lower serum TSH by about 0.3 mU/L.

• The effect may be dose-dependent.

• The effect disappears slowly after cessation of smoking.

• The effect is associated with an ambient amount of intake.

• The sympathetic nervous system activation may mediate the effect by stimulating the thyroid hormone synthesis and release.

• A slight rise of serum FT3 and FT4 accompanied the effect.

What Are the Concerns in Autoimmune Thyroid Disease?

Persistent inflammation caused by cigarette smoking may lead to the enlargement of the thyroid gland, which could raise concern in individuals having Hashimoto's or other Grave diseases.

• Graves Disease: Graves disease is autoimmune hypothyroidism characterized by thyroid enlargement, occurring twice as frequently in smokers as in nonsmokers. Moreover, in the people living with the disease, smoking was associated with increased disease progression, symptoms that worsen the eye disease, and a poorer response to thyroid treatment.

• Hashimoto's Disease: The association between Hashimoto's autoimmune disease and smoking was linked to hypothyroidism, which is not clearly defined. In addition, smoking further diminishes proper thyroid functioning and also enhances goiter development, particularly in iodine-deficient people.

• Goiter: Smoking may increase the risk of goiter irrespective of the number of cigarettes smoked. Based on the research and practice, the association is mostly seen in younger women and older adults. Younger women may tend to have diffuse enlargement of the thyroid, whereas older people have multiple goiters. For unknown reasons, smoking does not appear to be associated with a solitary thyroid nodule. The risk of having smoking-induced goiter is believed to be highest in the population with an iodine deficiency. In countries such as the Philippines, where goiter is more prevalent and even mild iodine deficiency is also endemic, nearly 28 percent of the population are smokers.

• Eye Problems: Effects of smoking and its impact on vision, most predominantly in people with Grave ophthalmopathy, a condition characterized by bulging and swollen eyes. A study shows that smokers with Grave disease are more likely to cause rapid eye deterioration, which includes the constriction of eye movement, the development of double vision, and irreversible optic nerve damage. In addition, Grave ophthalmology may appear for the first time after the treatment with steroids and radioiodine, which seems to be four times less effective in smokers than in nonsmokers.

• Thyroid Cancer: Thyroid cancer is common cancer in women. While smoking is a risk factor, as it is with lung and throat cancer. A study shows no difference in the indicine of the disease between women who smoked and those who did not.

• Vaping: Compared to tobacco and even cannabis, it has fewer effects on thyroid function. According to the research, some studies show withdrawal of nicotine is associated with reduced T3 and T4 Levels. Nicotine replacement through vaping or others may help in maintaining hormonal output in people with thyroid problems who are trying to quit smoking. Vaping is increasingly linked to lung illness. Thyroid hormone replacement therapy using Levothyroxine also appears to minimize nicotine withdrawal symptoms. Regular blood monitoring and an increased dosage of Levothyroxine are recommended for people who are on Levothyroxine and are planning to quit smoking.

Conclusion

Smoking will worsen thyroid symptoms, speed the progression of the disease, and cause thyroid cancer. In addition, smoking may increase the risk of metastasis and spread cancer beyond the tumor site to other body parts. Smoking has a distinct association with thyroid function and size in healthy subjects. Association with thyroid function in the autoimmune thyroid disease and thyroid function in autoimmune thyroid disease with thyroid size in nodular disease lowers the risk of thyroid carcinoma and higher the risk of non-toxic goiter.