title: Prospectively measured triiodothyronine levels are positively associated with breast cancer risk in postmenopausal women.
abstract: BACKGROUND
  The potential association between hypo- and hyperthyroid disorders and breast cancer has been investigated in a large number of studies during the last decades without conclusive results. This prospective cohort study investigated prediagnostic levels of thyrotropin (TSH) and triiodothyronine (T3) in relation to breast cancer incidence in pre- and postmenopausal women.
  
  
  METHODS
  In the Malmö Preventive Project, 2,696 women had T3 and/or TSH levels measured at baseline. During a mean follow-up of 19.3 years, 173 incident breast cancer cases were retrieved using record linkage with The Swedish Cancer Registry. Quartile cut-points for T3 and TSH were based on the distribution among all women in the study cohort. A Cox's proportional hazards analysis was used to estimate relative risks (RR), with a confidence interval (CI) of 95%. Trends over quartiles of T3 and TSH were calculated considering a P-value < 0.05 as statistically significant. All analyses were repeated for pre- and peri/postmenopausal women separately.
  
  
  RESULTS
  Overall there was a statistically significant association between T3 and breast cancer risk, the adjusted RR in the fourth quartile, as compared to the first, was 1.87 (1.12 to 3.14). In postmenopausal women the RRs for the second, third and fourth quartiles, as compared to the first, were 3.26 (0.96 to 11.1), 5.53 (1.65 to 18.6) and 6.87 (2.09 to 22.6), (P-trend: < 0.001). There were no such associations in pre-menopausal women, and no statistically significant interaction between T3 and menopausal status. Also, no statistically significant association was seen between serum TSH and breast cancer.
  
  
  CONCLUSIONS
  This is the first prospective study on T3 levels in relation to breast cancer risk. T3 levels in postmenopausal women were positively associated with the risk of breast cancer in a dose-response manner.
authors.affiliation: Department of Surgery, Skåne University Hospital Malmö, Malmö, Sweden. Ada.tosovic@med.lu.se
authors.forename: Ada
authors.initials: A
authors.lastname: Tosovic
authors.forename: Anne-Greth
authors.initials: AG
authors.lastname: Bondeson
authors.forename: Lennart
authors.initials: L
authors.lastname: Bondeson
authors.forename: Ulla-Britt
authors.initials: UB
authors.lastname: Ericsson
authors.forename: Johan
authors.initials: J
authors.lastname: Malm
authors.forename: Jonas
authors.initials: J
authors.lastname: Manjer
mesh_terms: D001943:Breast Neoplasms / Q000097:blood* / Q000473:pathology; D002285:Carcinoma, Intraductal, Noninfiltrating / Q000097:blood* / Q000473:pathology; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D009361:Neoplasm Invasiveness; D017698:Postmenopause / Q000097:blood*; D017697:Premenopause / Q000097:blood*; D011379:Prognosis; D011446:Prospective Studies; D011863:Radioimmunoassay; D012307:Risk Factors; D015996:Survival Rate; D013972:Thyrotropin / Q000097:blood; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin