Hypothyroidism as a Risk Factor for Cancer: A Systematic Review and Implications for Future Studies

Background: Hypothyroidism is one of the most common endocrine abnormalities and is often comorbid with additional types of cancer. There has been little study into the manner in which hypothyroidism affects future risk for these various types of cancer. Purpose: Summarize all published cases of cancer which describe hypothyroidism as a risk or protective factor for developing cancer. From there, we hope to lay groundwork for future study that could further characterize the effect of hypothyroidism on tumorigenesis. Data Sources: A broad PubMed search was performed on December 21, 2016, and published articles from March 1971 through the search date were included in the initial review. Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) were utilized. Study Selection: Titles were initially screened for relevance. For those that were relevant, full-text articles were reviewed, and studies describing the relationship between hypothyroidism and the development of cancer were included. Data Extraction and Synthesis: Extracted elements from each article include type of cancer described, number of cases, how hypothyroidism was assessed, possible confounders, and what, if any, association between hypothyroidism and the given cancer was observed. Data were consolidated and stratified by tumor type. Limitations: Limitations include the low number of studies examining the relationship of hypothyroidism with each tumor type, as well as the heterogeneity of how hypothyroidism was defined across studies. Conclusions: Hypothyroidism was associated with an increased risk of colorectal cancer and hepatocellular carcinoma, while it was associated with a decreased risk of prostate cancer. Its relationship with breast cancer was mixed. No association was observed with endometrial and ovarian cancers, and there is a possible relationship with parathyroid and esophageal carcinomas.