A growing body of data suggests that nicotine can have a detrimental effect on thyroid function. Animal research has established a cellular link between nicotine and thyroid activity. Human research also points to a possible connection between nicotine-induced altered thyroid function and cognitive impairment.

Nicotine may cause underactive or overactive thyroid symptoms in certain individuals. Reproductive-aged women, heavy smokers, and people attempting to quit smoking may be more susceptible to hypothyroidism. People already diagnosed with hyperthyroidism and the general population may more likely develop an increase in overactive thyroid symptoms.

Thyroid hormone levels should be closely monitored in smokers as well as in those attempting to quit so that abnormalities can be addressed. It may be that weight gain and other negative effects associated with smoking cessation can be minimized with thyroid hormone supplementation.