How a doctor spotted this woman's cancer while watching reality TV.

New York doctor Erich Voigt was watching a reality TV show in early May when he noticed something alarming about a woman on the screen.

Nicole McGuinness, a contestant on ‘Beachfront Bargain Hunt’, had a small lump on the left side of her throat.

Speaking to the New York Post on Tuesday, Dr Voigt said once he spotted the lump, he immediately went “from being relaxed to [thinking], gosh, she may have a medical problem.”

As an otolaryngologist, Dr Voigt had examined many patients with strikingly similar symptoms. As he watched Nicole, it was the lump’s “way of moving under the skin” which alarmed him.

"I thought, I don’t think she knows she has this. I felt obliged and sort of guilty, like I should let her know," he told The Post. "I paused my TV and rewound it. I had to make a choice whether to ignore it or actively try to contact her."

The TV show, however, had never mentioned Nicole's last name.

Feeling compelled to pursue his instincts, Dr Voigt shared a video from the show to Facebook, asking his friends for help in finding the woman's contact details.

"I am watching a tv show and notice this woman has a left thyroid mass. She needs a sonogram and fine needle biopsy," he wrote on Facebook. "I wonder if she knows and hope it's benign."

Two weeks later, he got in touch with 32-year-old Nicole McGuiness from North Carolina - a brain cancer survivor.

Nicole went to get the tests Dr Voigt recommended, and was diagnosed with thyroid cancer, which she is now receiving treatment for.

"It’s just a miracle, in my opinion, that he happened to see this on television,” she told ABC News. "I can't express how grateful I am."

Coincidentally, it's not the first time a viewer has helped a reality TV star spot thyroid cancer. In 2013, registered nurse Ryan Reade was watching a marathon of real estate show 'Flip or Flop' when he noticed contestant Tarek El Moussa had a lump on his neck. Reade contacted the show's producers, who passed the information on to Moussa.

Having experienced throat problems for years, Reade's warning was a "lightbulb moment" for Moussa. After seeing his doctor, he was diagnosed with stage 2 thyroid cancer.

A distinctive lump in the neck, it seems, is a symptom we should all look out for.