Sigh. Another junk academic article with a sensationalist title but with little substance.

tldr they found a correlation between people who both had some thyroid nodule and also answered their questionnaire on bluetooth usage. That's right, they asked people about their bluetooth usage and set out to find correlations with medical conditions.

No information on where sub-population sampling was done, age distribution, occupation, diet, exposure to all sorts of other stuff that can lead to the same medical condition, etc., etc. Questionnaires are already a very bad source of information, but it gets worse. They basically don't have good positive controls and essentially no negative controls. No discussion on how bluetooth frequencies compared to other much more damaging forms of radiation. I could go on. All the hallmarks of just plain bad science.