Will metformin use lead to a decreased risk of thyroid cancer? A systematic review and meta-analyses

Table 1 Characteristics of studies included in this meta-analysis

Study	Study design	Data source	Location	Sample size	Age	Study period	Assessment method of thyroid cancer	Estimate effect (95% CI)	NOS scores	Adjusted confounders
Becker 2015	Case–control	UK-based Clinical Practice Research Datalink	United Kingdom	8603	Not specified	From January 1995 to December 2014	NA	OR:1.42 (0.74–2.73)	7	Adjusted^a
Cho 2017	Cohort	Korean National Health Insurance claim database	Korea	256,906	Aged ≥ 30 years	From January 2005 to December 2009	ICD-10	HR:0.69 (0.60–0.79)	7	Adjusted^b
Luo 2016	Cohort	The Women’s Health Initiative in USA	United States	33,005	50–79 years old	From September 1993 to December 1998	Self-administered questionnaires	HR:0.99 (0.46–2.12)	6	Adjusted^c
Sulu 2022	Case–control	Database of Patients with Acromegaly in the own university hospital	Turkey	291	Above 18 years of age	From January 1983 to December 2019	Medical READ codes	OR:0.62 (0.67–0.83)	8	Adjusted^d
Tseng 2014	Cohort	National Health Insurance database	China	1,414,723	Not specified	From 1996 (the earliest database available) to 2009	ICD-9-CM	HR:0.43 (0.34–0.56)	8	Adjusted^e

^aAdjusted for other antidiabetic drugs in the table, BMI, smoking, diabetes mellitus, hyperthyroidism, and goiter
^bAdjusted for age, sex, year of study enrollment, income, living area, and antidiabetic drugs other than metformin
^cAdjusted for age at enrollment, ethnicity, education, smoking status, recreational physical activity, alcohol intake, history of HT use, and previous thyroid disease
^dAll variables are scaled to prevent range related bias in conditional logistic regression
^eAdjusted for age, the severity/duration of diabetes, other antidiabetic drugs used, hypertension, gender, other cancer, chronic diseases status, medication

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