Prospectively measured triiodothyronine levels are positively associated with breast cancer risk in postmenopausal women

Thyroid disorders and breast cancer both have a postmenopausal peak incidence, and a potential association between hypo- and hyperthyroid disorders and breast cancer has been investigated in a large number of studies during the last decades [1‐19]. However, the results have not been conclusive.

Experimental studies have shown that thyroid hormones can have estrogen-like effects in breast cancer, and that thyroid hormone receptors influence both normal breast cell differentiation and breast cancer cell proliferation [2, 3]. Several clinical and epidemiological cross-sectional studies have been performed comparing levels of triiodothyronine (T3), thyroxin (T4) and thyrotropin (TSH) in breast cancer patients versus healthy controls. The results have been contradictory, some in favor of higher levels in cases [4‐7] other in controls [8] and yet some report no differences [9‐15].

It is, however, difficult to conclude from cross-sectional studies whether differences in thyroid hormone levels are associated with different risks of breast cancer, or if breast cancer itself alters thyroid hormone levels. To date, there is only one prospective cohort study on this issue, involving 61 breast cancer cases, where pre-diagnostic levels of TSH and T4 were related to subsequent risk of breast cancer [16]. The present study is a population based, prospective cohort study including 2,696 pre and peri/postmenopausal women in whom TSH and T3 levels were measured at baseline. During a follow-up of a total of 51,989 person-years, 173 women were diagnosed with incident breast cancer.

The aim of the present study was to investigate prediagnostic serum levels of TSH and T3 in relation to breast cancer incidence in pre- and peri/postmenopausal women, respectively.

Advanced age, peri/postmenopausal status, use of OC, HRT and high BMI, were more common in high T3 quartiles (Table 1). With regard to TSH, the highest percentage of peri/postmenopausal women, were in the lowest and highest TSH quartiles, respectively (Table 2).

Overall, the risk of breast cancer was statistically significantly higher in the fourth T3 quartile as compared to the first quartile (Table 3). This association was even stronger for postmenopausal women, also showing a marked dose-response pattern (P-value for trend < 0.001). No such associations were seen in premenopausal women. There were no statistically significant interactions between menopausal status and T3 (P: 0.48) or TSH (P: 0.83).

There was no overall statistically significant association between serum TSH levels and breast cancer (Table 4).

In the analyses stratified for BMI, there was a weak, non-significant association between high T3 levels and breast cancer in the group with BMI over 25, the RR for the fourth vs. the first quartile was 2.41 (0.99 to 5.84), as compared to the corresponding RR in women with BMI below 25, 1.31 (0.72 to 2.41) (other data not shown).

Adjusted RRs were very similar to crude RRs with regard to both T3 and TSH. When women who developed breast cancer within three years after screening were excluded from the analyses, all results were similar for T3, but for TSH in premenopausal women, the risk in the second quartile was even stronger (Tables 3 and 4).

The analyses of combinations of low and high T3/TSH levels showed a statistically significant association for crude RR in the highT3/highTSH groups, as compared to the low/low category (1.62: 1.01 to 2.60). The association was, however, not statistically significant in the adjusted analysis (1.63: 0.99 to 2.69). The data were also stratified for TSH, below and above median with regard to T3 quartiles. There was a statistically significant risk for breast cancer in the highest T3 quartile (2.69: 1.19 to 6.08) in women with a TSH above the median. The corresponding RR in women with low TSH levels was 1.62 (0.68 to 3.86). There was no statistically significant interaction between T3 and TSH in the whole cohort, in premenopausal or peri/postmenopausal women.

When the analyses were repeated, excluding the women born in 1935, all results were similar, crude RR = 5.56 (1.66 to 18.57) for postmenopausal women in the highest T3 quartile (other data not shown).

Also, the analyses were repeated excluding women treated with HRT and OC, and the results were similar, crude RR = 6.50 (1.99 to 21.2) for postmenopausal women in the highest T3 quartile (other data not shown).

This is the first prospective study on T3 levels in relation to breast cancer risk. It shows that T3 levels in postmenopausal women are positively associated with the risk of breast cancer in a dose-response manner.

It may be asked whether breast cancer cases in this cohort are representative of the whole breast cancer population. This cohort mainly comprised middle-aged women and 70% of the women invited to the health examination attended. As we have no information about exposure to the studied risk factors in women outside this cohort, observed incidence rates may not be applicable to all age groups or to the general population. However, as there was a wide distribution of T3 and TSH levels, it was possible to make internal comparisons between subjects with low and high values, respectively. We consider that our estimations of relative risks were not considerably affected by selection bias.

Incomplete follow-up may affect the results. However, the Swedish Cancer Registry and the Swedish Cause-of-Death Registry have been validated and found to have a completeness of about 99% [25].

Subjects with elevated T3 levels and a possible diagnosis of hyperthyroidism may already be within the health care system and due to this the diagnosis of breast cancer might be established earlier than for euthyroid subjects. However, general mammography screening was introduced in Sweden already in 1986 and stands for the majority of detection of breast cancer cases [26]. It is unlikely that the thyroid status does affect participation in mammography screening, hence a possible detection bias in hyperthyroid breast cancer patients was probably not important. It should also be noted that most women with T3 levels in the fourth quartile in the present study still had levels within the normal range, and these women did probably not have any symptoms of thyroid disease.

The method of analysis for T3 remained the same throughout the study. Not all women in the study population had information on T3, but our analyses showed no difference in risk of breast cancer in relation to T3 when the group screened in 1990 was excluded. Hence, there was probably no major selection bias due to the inclusion of only some subjects with information on T3 in the later period.

The method of analyses for TSH changed between 1983 and 1990 as did the reference values. This was taken into account in our statistical analysis.

There is a well known circadian and seasonal variation concerning T3 and TSH levels [27]. Tracking of individuals, that is, ranking of individuals over time is, however, quite stable [28]. A true variation over time would most likely have led to a un-differential misclassification of T3 and TSH, and, hence, attenuated observed risks.

The use of total T3 as a marker of T3 status involves an important methodological aspect. Most T3 in the circulation is bound to three transport proteins, thyroxin binding globulin (TBG), transthyretin and albumin. An increased TBG concentration leads to higher levels of total T3, and this is important to consider in studies of breast cancer risk, as exogenous estrogens, that are risk factors for breast cancer, that is, HRT and OC, led to increased TBG binding capacity [29]. In order to exclude the association between total T3 levels and breast cancer risk as only an effect of differences in the use of HRT and OC, we adjusted our analyses for HRT and OC. Moreover, the analyses in postmenopausal women were repeated excluding women that had used HRT or OC. Following that exclusion, the RR for T3, as compared to the first quartile, was in the second quartile 3.11 (0.92 to 10.6), in the third 4.55 (1.35 to 15.3) and in the fourth quartile 6.50 (1.99 to 21.2). Hence, both adjustment for HRT/OC and exclusion of subjects using these medications resulted in similar risk estimates as were seen in the main analysis. This finding, together with the notion that there is no known association between endogenous estrogen levels and TBG [29], makes it unlikely that the results in the present study were caused by differences with regard to TBG levels.

The current study is the largest prospective study to date on T3/TSH levels and breast cancer risk, but the number of cases was limited. Confidence intervals were wide and the statistical power relatively low. This led to an imprecision in the estimates related to T3, and there is a risk that the lack of association between TSH and breast cancer was due to a type II error.

A large number of epidemiological studies have been performed during the last 50 years on benign thyroid conditions in relation to breast cancer [9‐14, 30‐32], and at least seven studies included more than 1,000 breast cancer cases [9‐14, 30]. However, most previous studies show no association between thyroid condition and breast cancer. A methodological problem is that the great majority of these studies were cross-sectional; hence it may be difficult to establish a potential causal relationship between thyroid disorders and breast cancer. A study by Cristofanilli et al., including 1,136 breast cancer cases, reported that women with previous hypothyroid conditions had an increased risk of breast cancer [30]. Another prospective study including a cohort of 2,775 women found that previous hypothyroidism and the use of thyroid medications was associated with an odds ratio of 3.8 for breast cancer [16]. An indirect evidence of an association between hypothyroidism and breast cancer is offered by studies following women treated for thyroid cancer (that is, by surgery or radio isotopes causing a hypothyroid state). For example, Brown et al. used the SEER registry and found an increased risk of breast cancer in women with a previous diagnosis of thyroid cancer [33]. These three studies were prospective, but a methodological problem is that the majority of these patients had been treated with thyroxin. That is, the results may have been confounded by exposure to exogenous thyroid hormones. Moreover, as the above studies used information on thyroid disorders usually obtained by interviews or record-linkage with diagnosis registries thyroid hormone levels were not directly measured. Thus, these studies do not rule out the possibility that sub-clinical hyperthyroid conditions, or even elevated T3 levels within the normal range, may play a role in breast cancer pathogenesis.

Some previous cross-sectional studies have found that high T3 levels are positively associated with breast cancer [1, 5, 7], among them the largest study to date including 226 cases (mean age 55 years, range 28 to 79) and 166 controls (mean age 46 years, range 17 to 65) [5]. Turken et al. examined 150 cases and 100 controls, and the results indicated that high T4 levels were associated with breast cancer [4]. On the other hand, several reports have suggested an association between low T3 [8, 24, 34] and/or low T4 levels [8] and breast cancer risk. Indeed, the only prospective study found a negative association between T4 levels and breast cancer risk [16]. Finally some studies did not show any associations at all with regard to T3/T4 and breast cancer [17, 35, 36], the two largest with 356 and 136 cases, respectively [35, 7]. High TSH levels have been associated with breast cancer risk in some studies [24, 37], but the only prospective analysis to date found a negative association [16]. Yet, most have found no association between TSH levels and breast cancer [3, 5, 7, 8, 35, 36, 38‐40]. One study found that TSH is low in cases among postmenopausal women but high in cases among premenopausal [1], indicating that menopause status may modify the potential association between thyroid hormones and risk of breast cancer. Autoimmune thyroid diseases (defined as the presence of thyroid peroxidase antibodies; TPOab) have been positively associated with breast cancer risk in several cross-sectional studies [4, 17, 40]. It is unclear whether the presence of TPOab in serum from patients with breast cancer is related to an increased risk following TPOab related conditions or if it is a general autoimmune response to the malignancy [41]. Indeed, the only prospective study showed that the pre-diagnostic presence of TPOab was not related to the subsequent risk of breast cancer [16]. In our prospective study, breast cancer cases diagnosed within three years after screening were excluded, hence the effect of (subclinical) cancer disease on thyroid hormones or TSH, that is, reverse causality, is unlikely.

The presence of thyroid hormone receptors in human breast and breast cancer cell lines has been established [42‐45], and the proliferative effect of T3 has been confirmed by various experimental studies on breast cancer. Our findings are in line with these data [1‐3, 46].

It has been shown that T3 binds and stimulates the estrogen receptor, acting in synergy with estrogen on breast cancer cell lines, potentiating the estrogenic effect and enhancing cell proliferation [47]. The role of estrogen in carcinogenesis of the breast is well known and the possibility that this effect may be even stronger in conjunction with high levels of T3 could in part explain the results in the present study.

Obesity is associated with increased breast cancer risk in postmenopausal women [48]. Estrogen levels are higher in obese compared to normal weight women. The potentiating effect of T3 might further increase the risk in this subgroup. Although our results did not reach statistical significance, they are in line with this hypothesis.

The significant positive association between T3 and breast cancer is specifically strong in postmenopausal women. Therefore, an imbalance between estrogen and T3 with an increased T3/E2 ratio may be more important than a pure synergistic effect between these two hormones for the risk of developing breast cancer. It has been suggested that this imbalance might enhance breast cancer development [1]. This theory is in accordance with our findings that higher T3 levels in postmenopausal women are positively associated with the risk of breast cancer in a dose-response manner. The present study excluded women with a previous treatment for goiter and there was no information on thyroxin use. An important issue for future studies will be to evaluate the widespread, long-term use of thyroxin treatment in TSH suppressive doses for benign thyroid disease [49, 50], and the management of women with subclinical hyperthyroidism [51‐53]. Such studies will contribute with important evidence whether or not to treat mild and/or sub-clinical thyroid disorders.

In conclusion, the present prospective cohort study, the first of its kind on prospectively measured T3 levels and breast cancer risk, indicates that high T3 levels in postmenopausal women are positively associated with the risk of breast cancer in a dose-response manner.