Objectives: The main objective is to investigate whether there is a correlation between anti-TPO, TSH, and FT4 and FT3 concentrations in patients with Hashimoto's thyroiditis who are euthyroid or subclinically hypothyroid in the development of the clinical picture of hypothyroidism. Study Design: a historic cohort study Patients and Methods: The sample consisted of 51 patients with Hashimoto's thyroiditis, followed in 2002. - 2013. The main criteria for patient involvement in the study were elevated anti-TPO and cytologically confirmed Hashimoto's thyroiditis. None of the patients were on substitution therapy, nor did they start taking substitution therapy after the first examination. The data was collected from the medical records of the Thyroid Diseases Department of Clinical Hospital Center in Osijek. Results: A weak positive but significant correlation between anti-TPO and TSH was observed, that is, the higher the anti-TPO values, the higher the TSH levels and vice versa. There is no significant correlation between anti-TPO and FT3. There is a weak, but insignificant, negative correlation between anti-TPO with FT4. Thirty-seven (73%) subjects did not develop the clinical manifestations of hypothyroidism, while the other 14 (27%) did. The median time to diagnosis compared to the first examination was 16.5 months. The TSH concentration (mIU/L) was higher at diagnosis compared to the first examination (Hodges -Lehmann estimator 6.45 mIU/L). Conclusion: The increased concentrations of TSH and anti-TPO in euthyroid and subclinically hypothyroid patients with Hashimoto's thyroiditis may serve as risk factors in the development of hypothyroidism. However, although there is a statistically significant positive correlation between these concentrations, it is unfortunately not strong enough to have a predictive value for the development of a clinical picture of hypothyroidism over a 10-year follow-up period in such patients. Additional simultaneous determination and concentrations of anti-Tg are recommended.