Thyroxine (T4) and Cancer

Introduction

Hypothyroidism has been “known” to be protective against cancer for many decades with suspected links between the thyroid and cancer going back over a century. This topic looks at recent research that reveals how T4 but not T3 promotes cancer by binding to the integrin αvβ3 receptor. T4 refers to all forms of thyroxine, from the thyroid or from tablets.

IMPORTANT: If you have cancer or are worried about it you should seek advice from your doctor or an oncologist. I have no cancer expertise whatsoever. The objective is to draw attention to the unique carcinogenic effects of T4 and review thyroid treatment so that cases of cancer and deaths are reduced.

While studying the effects of thyroxine on cancer we should keep in mind three questions: –

  1. How and to what degree does thyroxine promote cancer?
  2. How much illness and death is associated with levothyroxine monotherapy?
  3. How confident can we be with the data, what degree of certainty is there?

We will see how T4 acting on the integrin αvβ3 receptor promotes cancer by the following mechanisms: –

  1. T4 proliferates the growth and metastasis of cancer cells – helps cancer spread.
  2. T4 increases angiogenesis (formation of new blood vessels) – cancer cells need new blood vessels to grow.
  3. T4 inhibits apoptosis – inhibits the death of cancer cells.
  4. T4 enhances radioresistance – protects cancer from radiotherapy.

These effects are dose dependent.

Some studies look at the potential to treat cancer by lowering serum T4, or by blocking the ability of T4 to bind to integrin αvβ3 receptors. This provides good evidence. This topic does not discuss or advise on cancer treatment. This should be done by the oncologist in charge of the patient’s care.

Cancers can have profound effects on thyroid hormones so we only look at studies that compare thyroid hormone levels with cancer incidence several years later. These studies have limitations, the most obvious being that they look at hormone levels at a snapshot in time. This is likely to dilute the results if subjects with abnormal hormone levels are given corrective treatment. There is also the possibility that abnormalities might worsen and remain untreated.

There are many ways thyroid hormones can affect cancer risk so before we look at the integrin αvβ3 receptor we need to consider two exceptions: liver cancer and breast cancer.