The incidence of thyroiddisease is increasing dramatically worldwide, but it varies between countries.

Up to 20 million Americans were affected with some sort of thyroid disease in2014, and this number is expected to grow 1. Hypothyroidismis the most common type of thyroid disease whereas it is five to eight timemore in women than men 2. In the US, theprevalence of biochemical and clinically evident hypothyroidism is about 4.6%and 0.3% but it varies considerably by age group 3. Levothyroxine isa synthetic thyroid hormone, used to treat hypothyroidism and prevention ofvarious types of goiters.

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It deactivates abnormal thyroid tissue growth orfunction, and helps to maintain a clinical and biochemical euthyroid state inthyroid gland 4, 5. In recent year,concerns have been raised regarding the risk of levothyroxine medication forvarious types of adverse effects. Also, thyroid disease appears to pose agreater risk of breast 6, 7, pancreatic 8, and stomachcancer 9. Despite growing popularityfor the treatment of hypothyroidism, levothyroxine has further raised concernsregarding cancer risk. Cornelli et al.10 reported thatlevothyroxine is associated with increased risk of lung cancer; althoughlong-term use is associated with a reduction in risk of colorectal cancer 11. However, severalepidemiological studies were conducted which evaluated for an associationbetween levothyroxine use and cancer risk, but the results were inclusive.

Additionally,biological studies also showed an evidence that levothyroxine is linked tooverproduction of reactive oxygen species (ROS) which help to increase oxidativestress in the body 12, 13. Also, oxidativestress is considered as a main factor for various chronic conditions likecancer and autoimmunity.Due to the possible biologicalevidence and mixed results from epidemiological studies; We, therefore,conducted a nationwide population-based case-control to investigate whether thelong-term use of levothyroxine is associated with an increased risk of cancerwhen compared to non-levothyroxine users. To our knowledge, no study hasexamined the relationship between levothyroxine use and risk of differentcancer in a large population.


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