The aim of this paper is to show the role and importance of laboratory diagnostics in patients suffering from the autoimmune disease Hashimoto's thyroiditis. This diagnostic is the basis for establishing a diagnosis, as well as for monitoring and prognosis of this disease, and along with some other methods such as ultrasound, it also provides safe and reliable results. The paper first deals with describing the organs of the thyroid gland and explains what kind of disease Hashimoto's thyroiditis is, thereby introducing the importance of the topic of the paper itself. For diagnosis, biochemical tests were performed on the concentrations of the most important thyroid hormones, TSH, T4, fT4, T3 and fT3. They were measured on a Roche cobas e 800 device using the "ECLIA" electrochemiluminescent immunoassay method. The amount of antibodies against thyroglobulin (anti-Tg) and antibodies against thyroid peroxidase (TPO) were measured on an Abbott-Alinity device using the chemiluminescent immunochemical method "CMIA". The alleles of the HLA-B and HLA-DRB1 loci were also determined by the molecular method PCR-SSO (Polymerase chain reaction sequence-specific oligonucleotide) on a Luminex® device. For the purpose of typing the patient in order to determine the potential genetic basis of the disease. The results showed extremely high concentrations of antibodies against thyroglobulin (anti-Tg) and antibodies against thyroid peroxidase (TPO), but the typing results did not indicate a genetic predisposition to the disease in this patient.