Introduction: The thyroid gland is an endocrine gland that plays a significant role in the regulation of metabolism, growth, and serum electrolyte concentrations. An increasing number of people are suffering from thyroid diseases, which are categorized according to function as hypothyroidism and hyperthyroidism. Objectives: To examine the level of knowledge of health professionals about hypothyroidism and hyperthyroidism. To examine the differences in the level of knowledge of health professionals about hypothyroidism and hyperthyroidism in relation to gender, age, level of education, years of service, and in relation to the level of health care at which the respondents are employed. Methods: The research was conducted on the principles of a cross-sectional study. The survey questionnaire was created for research purposes using a Google form and was distributed via social networks in September 2022. The respondents are adults employed in the healthcare system in the Republic of Croatia. The conducted research is voluntary and anonymous, it took 10 minutes to fill out the questionnaire. Results: The level of knowledge of health professionals about hypothyroidism and hyperthyroidism is good. Female respondents show a significantly higher level of knowledge compared to male respondents. Respondents with 30 and more years of work experience show significantly better knowledge compared to respondents with 16 to 30 years of work experience and 15 and less years of work experience. Conclusion: The level of knowledge about hypothyroidism and hyperthyroidism increases in correlation with the increase in years of service, which shows the importance of the experience of health professionals in providing health care to patients with hypothyroidism and hyperthyroidism. Based on this research, a good level of knowledge of health professionals about hypothyroidism and hyperthyroidism can be observed, but additional education is needed in order to achieve high quality in the healthcare