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Background: Each toxic agent results in unique presentations, depending on what neurophysiological changes occur following exposure. Scientific understanding of lead toxicity in the organ systems and at low levels of exposure continues to evolve. However, effects of lead poisoning on the thyroid gland function are controversial. In this descriptive study, changes in thyroid hormones in patients with lead poisoning were compared with patients with opioid addiction and healthy individual who were matched for age and sex. Methods: In this descriptive study, which was conducted from March 2016 to February 2017 in Loghman Hakim Hospital in Tehran, ninety patients were evaluated using convenient sampling method for lead and thyroid hormones levels. The levels of lead and thyroid hormones were evaluated using standard laboratory method, in thirty male patients with lead poisoning, thirty male patients with opioid addiction referring to addiction treatment center and thirty healthy male referring to a blood donation center who were matched for age. Thyroid function parameters in patients with lead toxicity were compared with those of control groups. Results: The lead level in the lead toxicity group was 57.5±23.5 µg/dl, in the first control group with opium addiction 7.8±3.8 µg/dl and in the healthy male referring to a blood donation center, 5.9±9 µg/dl. Thyroid stimulating hormone (TSH) was significantly lower in patients with lead poisoning (0.2±0.01 Milli-International Units Per Liter (mIU/L) than in the patients with opioid addiction (2.4±1/05 mIU/L) and healthy male referring to a blood donation center (2.3±0.01 mIU/L) (P= 0.04), and thyroxine hormone (T4) was significantly higher in patients with lead poisoning (17.8±2.6 µg/dl) than in the patients with opioid addiction (8.8±1/9 µg/dl) and healthy male referring to a blood donation center (7.4±3.5 µg/dl) (P= 0.02). Conclusion: Lead toxicity has an effect on thyroid function and it reduces thyroid stimulating hormone and increases thyroxin levels. Clinicians should be aware of the potential hazardous effects of lead on the thyroid and mechanisms through which lead causes these effects on thyroid function need to be elucidated.