Study of Euthyroid Hypothyroxinemia in Metastatic Breast Carcinoma

It is estimated that there are approximately 155,000 living with metastatic breast cancer in the US and the number is estimated to increase over the next years (SEER data). Although their median survival has improved over the last 2 decades from 17 months to approximately 24 months attributed to newer treatments, there is an ongoing need for additional strategies and research to improve survival and quality of life.

Many studies have explored the connection between hypothyroidism and hyperthyroidism and breast cancer with varied results ranging up to one third prevalence. Low Triiodothyronine (T3) and elevated Thyroid-Stimulating Hormone (TSH) levels have been detected in newly diagnosed breast cancer patients. Other studies have suggested that some of the common symptoms reported by breast cancer survivors such as fatigue and depression can be attributed to subclinical hypothyroidism.

L-thyroxine (T4) is the most commonly prescribed agent for the management of hypothyroidism in the US. However, there are data suggesting that T4 is a potent pro-oncogenic agent. Proposed mechanisms include stimulation of mitogenesis, angiogenesis and resistance to apoptosis, opposition of anti-PDL-1 and radiation effects. It has been postulated that the avbeta3integrin that is universally expressed on cancer cells harbors a thyroid hormone receptor and T4 interacts with it.

Triiodothyronine (T3) on the other hand, is significantly less oncogenic and less mitogenic and is downstream of T4 which is a T3 pro-hormone. Therefore, exogenous supplementation of T3 would decrease the T4 levels creating the desired state of euthyroid hypothyroxinemia.

The rationale of this study is to replace L-thyroxine (T4) with Triiodothyronine (T3) in hypothyroid patients with metastatic breast carcinoma while they continue to receive standard systemic therapy, titrating the dose to achieve a state of euthyroid hypothyroxinemia which is turn would result in a lower risk of disease progression and improved survival by lowering the concentration of T4.