Thyroid hormones help control energy, metabolism, and many of the body’s basic functions. While genetics play a significant role in how the thyroid works, everyday lifestyle habits and environmental exposures can also affect thyroid hormone levels.
Smoking is one of the clearest examples—it tends to lower TSH and raise T3 and T4, which can make the thyroid appear more active.
People with higher body weight often have higher TSH and higher free T3 levels, suggesting that weight and thyroid function influence each other. Eating too much iodine, especially from seaweed or supplements, can push TSH higher and lower thyroid hormone levels.
Environmental chemicals can also influence thyroid function. Perchlorate shows the strongest and most consistent effect by blocking the thyroid’s ability to absorb iodine, which can reduce thyroid hormone levels.
Other chemicals—such as PCBs, flame retardants, plastics, pesticides, and heavy metals—may also interfere with thyroid hormone levels, but results vary across studies. These mixed findings come from differences in exposure levels and the wide range of chemicals people encounter.
Overall, lifestyle choices and environmental exposures can cause slight changes in thyroid hormone levels even in people without thyroid disease. Knowing how these factors affect the thyroid can help people understand unusual or borderline test results and make informed decisions about diet, habits, and environmental risks.
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