A thyroid problem could help doctors understand how to beat breast cancer.
Some cancers react to oestrogen
Scientists at the University of Texas have discovered that women with underactive thyroid glands have a lower risk of breast cancer than others.
Thyroid hormones and the female sex hormone oestrogen share similar pathways in the body, say the researchers in the journal Cancer.
The breast cancer drug tamoxifen works by blocking oestrogen. Blocking thyroid hormones may be another way, they say.
The thyroid gland sits in the neck and produces hormones to regulate the body's metabolism.
If it becomes overactive, called hyperthyroidism, or underactive, hypothyroidism, the person will feel ill.
A person who is hyperthyroid can experience weight loss, a rapid heartbeat, excessive sweating and anxiety.
Conversely, someone who is hypothyroid might gain weight, become constipated and develop a swollen face and puffy eyes.
There has been conflicting views about the effect of thyroid hormones on the breast.
Some studies have linked high levels of thyroid hormones with breast cancer while others have reported a protective effect.
Dr Massimo Cristofanilli's team compared the medical records from 1,136 women with breast cancer and 1,088 healthy women attending their breast screening clinic in Texas.
They found women with hypothyroidism had a 61% lower risk of developing invasive breast cancer.
When they did develop breast cancer, it was typically diagnosed at an earlier stage and when the tumour was smaller in size.
Women in the breast cancer group were also 57% less likely to have hypothyroidism than the healthy women.
Dr Cristofanilli said: "It may be possible to design a treatment that specifically and narrowly targets thyroid hormone receptors, which might provide enough influence on the target cells to help prevent breast cancer - perhaps even serving as a complement to tamoxifen."
A spokeswoman for Breakthrough Breast Cancer said: "The influence of thyroid gland disease on breast cancer has been debated for some time.
"More research is needed into this potential connection and we look forward to the publication of further studies in this interesting area."