Examination of a 6-year-old boy revealed enlarged lymph nodes in both submandibular and cervical regions. Objectively: the 75, 84 and 85 teeth are decayed, there are presentations of commissural cheilitis. According to the boy's mother, he has been quickly getting tired, sweating from the least physical exercise, complaining about weakness throughout the last 2-3 months. He also gave up training in a sports class. What plan of additional examination should be adopted?