Nowadays, the number of thyroid gland disorders is rapidly rising. Laboratory together with ultrasound diagnostics has a key role in diagnosis, diferentiation of disorders and therapy monitoring. Thyroid function tests TSH, tT4, tT3, fT4, fT3, anti-TPO i anti-TG are determined for this purpose. In the routine work of the laboratory, it is sometimes necessary to postpone or subsequently determine certain tests, which is why it is important to know the time frames and temperature conditions in which the tests are stable. The aims of this study were to study the stability of thyroid function tests in serum in tubes with gel during 8 h at room temperature and during 72 h at 2 – 8 ºC, with previous storage of serum samples for 8 h at room temperature. The study was conducted in the Endocrine Laboratory of KBC Sestre milosrdnice, and the research design was adapted according to the instructions of the Working Group for Preanalytics of EFLM. The concentrations of TSH, tT4, tT3, fT4, fT3, anti-TPO and anti-TG were determined on the Abbott Architect i2000 automatic analyzer at defined time points (0 h, 4 h, 8 h, 24 h, 48 h and 72 h). The stability of an individual test is determined by comparing the mean percentage of difference (PD̅̅̅̅) at defined time points and the maximum allowed difference (MPD/%). The results of the study are presented graphically. The obtained results showed that, for any functional test, the mean percentage of difference at the defined time points did not exceed the maximum allowed difference, which proves that the functional tests are stable in serum in test tubes with gel for 8 h at room temperature and for 72 h at a temperature of 2 – 8 ºC with previous storage of the serum samples for 8 h at room temperature. Therefore, the determination of thyroid function tests in serum can be postponed or subsequently performed within 72 hours.