Does Hypothyroidism Impact Risk of Breast Cancer Recurrence or Death?

Patients who have hypothyroidism at the time of a breast cancer diagnosis or who develop hypothyroidism later on do not have an increased risk of breast cancer recurrence or death, according to a population-based cohort study recently published in Breast Cancer Research. The findings may serve to reassure breast cancer survivors that a diagnosis of hypothyroidism is unlikely to affect disease recurrence or survival.

“This is a well-done, large population cohort study that answers an important question examining any correlation with hypothyroidism and breast cancer recurrence,” Shannon Puhalla, MD, medical oncologist/hematologist at UPMC Hillman Cancer Center, told Cancer Network. Puhalla agreed with the authors’ conclusions, noting that this study does show that “having hypothyroidism either before or after breast cancer doesn’t affect recurrence.”

The study cohort was derived from the Danish Breast Cancer Group (DBCG) clinical database and included 35,463 women aged 35 years or older with stage I to III operable breast cancer. All women included in the cohort were diagnosed with breast cancer between 1996 and 2009. Women with a diagnosis of hyperthyroidism were excluded from the cohort.

At the time of breast cancer diagnosis, 1,272 women (4%) had hypothyroidism and 34,191 (86%) had healthy thyroid function; women with hypothyroidism were followed for a median of 5.6 years and women with healthy thyroid function were followed for a median of 6.0 years. During follow-up, 859 women (2%) who initially had healthy thyroid function developed hypothyroidism at a median of 3.4 years of follow-up.

Breast cancer recurred among 184 women (14%) who had hypothyroidism and 5,626 women (16%) with healthy thyroid function at the time of breast cancer diagnosis. In an adjusted analysis that considered confounders, the risk of breast cancer recurrence among women with hypothyroidism at the time of breast cancer diagnosis was similar to that of women with healthy thyroid function (hazard ratio [HR], 1.01; 95% CI, 0.87–1.19). The risk of breast cancer recurrence among women who develop hypothyroidism during follow-up was also similar to that of women with healthy thyroid function (HR, 0.93; 95% CI, 0.75–1.16).

A total of 10,094 women died from any cause during follow-up, 398 (3%) of which had hypothyroidism at the time of breast cancer diagnosis, and 274 (3%) who developed hypothyroidism later on. In an adjusted analysis, women with hypothyroidism at the time of breast cancer diagnosis did not have an increased risk of death (HR, 1.02; 95% CI, 0.92–1.14). A similar lack of association was seen for women who developed hypothyroidism later on (HR, 1.08; 95% CI, 0.95–1.23).

“[The study] shows that symptoms of hypothyroidism are potentially under-recognized and can overlap with side effects of cancer treatment, so it is important to keep hypothyroidism in the differential,” Puhalla said. “However, if hypothyroidism is diagnosed, then patients and physicians can be reassured that the impact on cancer recurrence is not concerning. Both breast cancer and hypothyroidism are common in women, so it’s reassuring that the incidence and prevalence of both conditions do not appear to be correlated.”