American researchers have found that a hormone produced by the human body can extend the lifespan of mice by 40 percent. Boosting levels of the hormone in humans could bolster immune function in the elderly and prove beneficial to cancer patients or to people suffering from chronic illnesses such as Type 2 diabetes.

A recent study from the Yale School of Medicine has found that cells in the thymus gland produce large quantities of a hormone called FGF21. The thymus gland is located just under the sternum, and plays a key role in the immune system by producing a type of white blood cell called T-cells. As we age, the thymus gland becomes fatty and loses its ability to produce T-cells, in turn increasing the risk of infection and of developing cancer in old age.

Researchers investigated the effects of both high and low levels of the hormone in mice approaching the end of their lifespans. They found that increased levels of FGF21 prevented the thymus gland from weakening and increased its capacity to produce new T-cells.

According to Vishwa Deep Dixit, professor of comparative medicine and immunobiology at Yale School of Medicine, who led the study, this discovery suggests that "elevating the levels of FGF21 in the elderly or in cancer patients who undergo bone marrow transplantation may be an additional strategy to increase T-cell production, and thus bolster immune function."

Further studies plan to investigate whether a drug developed with high levels of FGF21 could prolong human life expectancy and reduce the risk of illness caused by age-related immune system deficiency.

The FGF21 hormone is also produced in the liver and has been linked to weight loss. It improves the body's response to insulin and allows fat to be burned when glucose levels are low. It's therefore being studied for its therapeutic properties for sufferers of Type 2 diabetes.

Plus, the hormone's weight-loss properties are to be further investigated. "We will also look to developing a way to mimic calorie restriction to enhance immune function without actually reducing caloric intake," said Dixit.