Low levels of thyroid hormone may reduce the risk of breast cancer: evidence from a meta-analysis

Background: At present, the relationship between hypothyroidism and the risk of breast cancer is still inconclusive. This meta-analysis was used to systematically assess the relationship between hypothyroidism and breast cancer risk, and to assess whether thyroid hormone replacement therapy can increase breast cancer risk. Methods: The relevant articles about hypothyroidism and the risk of breast cancer were obtained on the electronic database platform. Relevant data were extracted, and odd ratios with corresponding 95% confidence intervals(95% CI) were merged using Stata SE 12.0 software. A total of 19 related studies were included in the meta-analysis, including 6 cohort studies and 13 case-control studies. Result: The results show that hypothyroidism can reduce the risk of breast cancer(odd ratios= 0.90, 95% CI 0.77-1.03), but in Asian populations, patients with hypothyroidism have an increased risk of breast cancer(odd ratios=1.17, 95% CI 0.98-1.35). In addition, patients who received thyroid hormone replacement therapy had a lower risk of developing breast cancer(odd ratios=0.87, 95% CI 0.65-1.09). Conclusion: Hypothyroidism and thyroid hormone replacement therapy reduces the risk of breast cancer, suggesting that low levels of thyroid hormone may be beneficial to breast cancer prevention. Due to the limited number of studies included more large-scale, high-quality, long-term prospective cohort studies are needed.

were obtained on the electronic database platform. Relevant data were extracted, and odd ratios with corresponding 95% confidence intervals(95% CI) were merged using Stata SE 12.0 software. A total of 19 related studies were included in the meta-analysis, including 6 cohort studies and 13 case-control studies.
Result: The results show that hypothyroidism can reduce the risk of breast cancer(odd ratios= 0.90, 95% CI 0.77-1.03), but in Asian populations, patients with hypothyroidism have an increased risk of breast cancer(odd ratios=1.17, 95% CI 0.98-1.35). In addition, patients who received thyroid hormone replacement therapy had a lower risk of developing breast cancer(odd ratios=0.87, 95% CI 0.65-1.09).
Conclusion: Hypothyroidism and thyroid hormone replacement therapy reduces the risk of breast cancer, suggesting that low levels of thyroid hormone may be beneficial to breast cancer prevention. Due to the limited number of studies included more large-scale, high-quality, long-term prospective cohort studies are needed.

Background
As a global public health problem, breast cancer has an increasing incidence on a global scale [1]. According to the 2017 US cancer statistics, breast cancer has become the most common malignant tumour in women, with about 250,000 new cases each year, accounting for 30% of new malignant tumours in women [2]. Therefore, the identification of risk factors for breast cancer and the adoption of effective early prevention and intervention measures are of great significance for breast cancer prevention and treatment.
The physiology and pathology of the breast are closely related to the endocrine of the body [3]. As the largest endocrine organ in the human body, the thyroid gland has specific regulation effects on various hormone levels and cell growth and development in the body, which brings new enlightenment to the research of breast cancer [4][5][6]. In 1976, Kapdi et al. first proposed that hypothyroidism increases the risk of breast cancer and is a risk factor for breast cancer [7]. Since then, many scholars have studied the relationship between hypothyroidism and the risk of breast cancer. However, the relationship between the two diseases remains controversial. Some studies have shown that hypothyroidism increases the risk of breast cancer [7][8][9]. Some studies have shown that hypothyroidism reduces the risk of breast cancer [10]. Besides, some studies have found no correlation between thyroid disease and breast cancer [11]. Therefore, whether hypothyroidism can increase the risk of breast cancer is worthy of further study.
Two meta-analyses have previously been studied for hypothyroidism and breast cancer risk [11,12]. Based on previous research, combined with the latest research, we have included more prospective studies and Asian population studies to assess the relationship between hypothyroidism and breast cancer risk systematically. In addition to this, explore the impact of thyroid hormone replacement therapy on breast cancer risk. 4

Search strategy
The Meta-analysis was performed according to the Preferred Reporting Items for Systematic reviews and Meta-analysis (PRISMA) statement [13]. Relevant clinical literature was extracted by systematic retrieval of PubMed(Medline), EMBASE, Springer, Web of Science, and Cochrane Library electronic databases up to date to October 2019. Our search strategy included terms for: "thyroid dysfunction" or "hypothyroidism" or "HT" and "thyroid diseases" or "breast cancer" or "BC" or "breast neoplasms" or "mammarmy cancer" and "risk" or"incidence." At the same time, we manually screened out the relevant potential literature in the references extracted.

Inclusion and exclusion criteria
The inclusion criteria: (1) Types of studies: Published studies exploring the relationship between hypothyroidism and breast cancer risk; (2) Subject: Female; (3) Exposure factors: Primary hypothyroidism, the diagnosis needs to be based on the detection of thyroid function; (4) Outcome indicators: the occurrence of primary breast cancer.
The exclusion criteria: (1) Non-primary hypothyroidism due to other causes; (2) Non observational studies; (3) Insufficient information was provided or no full-text; (4) Unable to obtain full text or quality assessment of the literature; 5 (5) Only the research with higher methodological quality is maintained for the analysis with repeated publication or data overlap.

Data extraction and Quality Assessment
Two researchers separately conducted literature screening, data extraction, and literature quality evaluation, and any differences could be resolved through discussion or a third inspector. Information secured from the enrolled literature included: first author's surname, year of publication, country of the population, sample size, follow-up time, outcome index, and corresponding OR value.
The Newcastle-Ottawa Scale (NOS) was used to assess the quality of the study from three aspects: cohort selection, cohort comparability, and outcome evaluation [14].
NOS scores of at least six were considered high-quality literature. Higher NOS scores showed higher literature quality.

Statistical analysis
All data analysis was performed using Stata12.0 software. The included OR and 95%CI were treated with the combined effect size. After that, the heterogeneity test was conducted. When p ≥ 0.05 or I 2 < 50% was performed, it meant that there was no apparent heterogeneity, and the fixed-effect model should be applied for a merger. When p < 0.05 or I 2 ≥ 50% indicated high heterogeneity, the random-effect model was applied. Combined effect size, if OR > 1 indicates that hypothyroidism is an unfavorable factor for breast cancer.If OR < 1 is the opposite. Begg's funnel plot was used to research publication bias detection. If OR < 0.05 indicates obvious publication bias. A total of 367416 samples from 19 studies involving were enrolled in this metaanalysis [4,[8][9][10][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. 6 cohort studies and 13 case-control studies were included in the study. Twelve articles were studied in the European population, five in the North American population, and two in the Asian population. All articles are of high quality because of NOS score no less than 6. The chief characteristics of the enrolled materials are detailed in Table 1.  Fig. 2a).

Subgroup analysis of hypothyroidism and risk of breast cancer
To further explore the relationship between hypothyroidism and breast cancer risk, subgroup analysis was conducted from three aspects: study type, population distribution, and follow-up time. In the subgroups with a follow-up date of no more than four years, the case-control study, and the North American study, the results showed that the risk of breast cancer in patients with hypothyroidism decreased by 20%, 15%, and 14%, respectively ( Fig. 3). But in Asian patients with hypothyroidism, the risk of breast cancer increases by 17% (OR 1.17, 95% CI 0.98-1.35; p < 0.001) ( Fig. 3a).
Relationship between thyroid hormone replacement therapy and breast cancer risk A total of 10 studies reported the relationship between the use of thyroid hormone replacement therapy and the risk of breast cancer [4,8,9,16,18,22,24,26,27].
Publication bias Figure 5A shows a funnel plot of the 20 articles included in this meta-analysis. The Begg test (Pr = 0.529) and the Egger test(P = 0.892) were used to detecting 9 publication bias showed that there was no possibility of publication bias. As shown in Fig. 5B, there were no publication biases in the ten articles on the study of thyroid hormone replacement therapy. The Egger test was P = 0.672, and the Begg test (Pr = 0.858).

Sensitivity analysis
The results of sensitivity analysis are generally stable, and the primary source of heterogeneity is in the research of Cristofanilli et al [24]. (Fig. 6a). So we excluded the literature of Cristofanilli and analyzed the other studies. The results revealed that the hypothyroidism could reduce the risk of breast cancer (OR:0.96 95%CI:0.92 -1.00, P < 0.001), and there was no heterogeneity(I 2 = 0, P = 0.577) ( Fig. 2b).
As shown in Fig. 6b, we can see that the source of heterogeneity is the Cristofanilli and Sandhu articles [24,26]. Therefore, we removed the two articles and reexecuted the meta-analysis. With no obvious heterogeneity (I 2 =0, p = 0.922) among these studies, so a fixed-effect model was used for assessment. The results show that patients taking thyroid hormone replacement therapy have a lower risk of breast cancer(OR:0.96 95%CI:0.83-1.08, P < 0.001 ( Fig. 4b).

Discussion
In recent years, the incidence of breast cancer has been on the rise worldwide.,which has become a major public health problem in today's society. The prevention and treatment of breast cancer has become an important task in the prevention and treatment of malignant tumors. Therefore, understanding the risk factors of breast cancer and facilitating the implementation of effective prevention and early intervention, is of great significance for the effective prevention and treatment of breast cancer.
More than 100 years ago, Beatson et al. used thyroid extracts to treat patients with metastatic advanced breast cancer, and the condition was significantly alleviated, sparking interest in exploring the relationship between thyroid and breast cancer [30]. Subsequent, a prospective study enrolled 2,738 women, and 61 women with breast cancer during follow-up showed that low serum free thyroxine levels increased the risk of breast cancer [8]. In 2016, a prospective cohort study of 61,873 women with hypothyroidism and 80,343 hyperthyroidism found that hypothyroidism slightly reduced the risk of breast cancer [10]. However, a prospective cohort study of 89,731 women with autoimmune hypothyroidism and 89,731 women with normal thyroid function indicated that autoimmune hypothyroidism was not associated with breast cancer risk [26]. Besides, some animal experiments also reflect the relationship between the two [31,32]. Animal experiments by López Fontana et al.
found that hypothyroidism mice inhibit the development of breast cancer and promote the apoptosis of breast cancer cells due to the low expression of β-chain protein and activation of the apoptotic pathway on the tumour cell membrane [31].
Due to the inconsistency of the above conclusions, we performed a meta-analysis to evaluate the relationship between hypothyroidism and breast cancer risk.

Main Findings
A total of 19 studies were included in this meta-analysis, and the results showed Therefore, we suspect that low levels of thyroid hormone may reduce the incidence of breast cancer. Our meta-analysis results also confirm the above conjecture.

Limitations
There   Relationship between thyroid hormone replacement therapy and breast cancer risk 24 Figure 5 Publication bias assessment 25 Figure 6 Sensitivity analysis