Frostbite Plagued Kate Upton After Sports Illustrated Photoshoot -- Why Is It Dangerous?

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Sports Illustrated cover model Kate Upton suffered from frostbite from her photoshoot in Antarctica, she revealed today on NBC's TODAY show.

"When I came back, I was losing hearing and eyesight because my body was shutting down, it was working so hard," the 20-year-old said on TODAY. Upton posed in little more than a swimsuit and a hooded jacket in many of the photos in the magazine's swimsuit edition.

Frostbite occurs when the skin and tissue freeze because of low external temperatures. If the affected areas aren't attended to right away, serious damage can occur that may necessitate the amputation of that part of the body, according to the Ohio State University Wexner Medical Center. The National Institutes of Health reported that frostbite becomes irreversible if the blood vessels are affected -- once that happens, gangrene (which is when the tissue dies) occurs and then amputation is necessary.

Symptoms of frostbite include feeling "pins and needles" and then numbness in the frozen area, or aching in the area, according to the National Institutes of Health. Thawing of the frozen part of the body may induce pain. The affected part of the body may also feel hard and appear grayish in color.

Frostbite often happens in tandem with hypothermia, which is a condition when the body is not producing heat fast enough to keep up with the heat it is losing because of cold temperatures, the Mayo Clinic reported. When a person has hypothermia, different vital organs begin to stop working normally -- which can eventually be fatal.

Failing eyesight is a sign of hypothermia, the Weather Channel reported -- a symptom Upton mentioned having.

“There are several stages of hypothermia and when it becomes severe blood vessels in the extremities, including the eyes, constrict to conserve energy and keep it near the body’s vital organ," Dr. Rupe Hansra, the senior director of eye care at LensCrafters who was not involved in Upton's care, told HuffPost. "It is alarming that Kate Upton found herself in such an advanced stage of hypothermia, but her vision should make a full recovery."