Abstract: Objective To analyze retrospectively the performance of thyroid hormone tests in the Bio-Rad external quality assurance system （EQAS） so as for continuous improvements of laboratory performance. Methods Data of total triiodothyronine （TT3），total thyroxine （TT4），free T3 （FT3），Free T4 （FT4） and thyroid stimulating hormone （TSH） from 12 samples in the ninth cycle were analyzed by Bio-Rad EQAS. And Z-score，mean Z-score and L-J distribution histogram at different concentration levels in the peer and method groups were evaluated with Beckman Coulter DXI 800 analytical system. Continual improvements were made for reliable performance. Results Samples No.6 of FT3，No.6 of FT4 and No. 3 of TT4 were warned due to their Z-scores between 2 and 3. And performance of sample No.5 of FT3 could not be accepted because its Z-scores were above 3. The average Z scores of TT3，TT4，FT3，FT4 and TSH in the 9th cycle were below 2 although that of FT3 was inferior to 97% of participants. Our laboratory performance was better in the 10th cycle than in the 9th cycle after continual improvements. Conclusion Further improvements would be necessary for the quality assurance of thyroid hormone analysis in personnel，environment，instrument detection system and sample processing.

Key words: Thyroid hormone , External quality assessment , Z score