Objectives: The main aim of our research was to describe in detail the group of individuals with HT regarding collected/measured phenotypes. The next objective was to determine whether there are differences in medians of HT-specific phenotypes among individuals receiving levothyroxine therapy and those who do not receive therapy. Finally, we aim to determine the correlation between phenotypes characteristic for HT and other significant phenotypes and, also, a correlation between HT-characteristic phenotypes. Design: Cross-section study Settings: Department of Medical Biology, University of Split School of Medicine Materials and methods: For the purposes of this graduate thesis we obtained the data from the HRZZ under the HRZZ Installation Grant "Genome-wide association analysis of Hashimoto's thyroiditis". HT patients were recruited after examination in Outpatient clinic for thyroid disorders at Department of Nuclear Medicine, University Hospital Split. The information on medical status of subjects, clinical classification of goitre, thyroid ultrasound, therapy, and levels of hormones and thyroid antibodies (TSH, T3, T4, fT4, TgAt, TPOAt) were used. Thyroid hormones (TSH, T3, T4, fT4) and antibody levels (TgAt, TPOAt) in plasma of participants were determined by radioimmunoassay method. All quantitative phenotypes are described by median and span, while for categorical phenotypes the number of subjects and the percentage of subjects from particular category are shown. Differences in medians were tested using non-parametric, Mann-Whitney test. Spearman's correlation test was used to test the correlation of observed variables. Results: We have characterized 405 subjects in this study, focusing on hormone levels, thyroid antibodies, anthropometric characteristics and smoking habits. The most common symptom of hypothyroidism in our study is nausea (64,7%), and the most common comorbidity is allergy or asthma (15,5%). Most of the subjects (85.8%) have a diffuse thyroid disease, while 57.7% of respondents belong in category 1a, which corresponds to palpable thyroid that is not visible at the stretching of the neck. We also noted significant differences in laboratory values of hormone levels and antibodies between respondents on therapy and non-therapeutic subjects. As the most significant finding in our study, we highlight the positive correlation between TgAt and the number of symptoms. Other results obtained are consistent with the literature. Conclusion: We performed a detailed characterization of the group of patients with HT, resulting with additional evidence and confirmations of findings from previous studies. We are the first to point to the newly discovered correlation between TgAt and the number of symptoms. Through our work we have contributed to better understanding of HT's clinical image and pointed out the prognostic value of antibodies with respect to disease severity.