BACKGROUND: Plastic tubes have recently been used for blood collection tubes in clinical laboratories. The silicone coated plastic tube is said to be very similar with the glass tube and to show no difference with the glass tube in routine blood test except for some tests such as hormone tests and drug monitoring. So, we investigated the influence of plastic tube on the thyroid hormone test using two types of plastic vacuum tubes. METHODS: A total of 105 cases for the total triiodothyronine (T3), total thyroxine (T4), and thyroid stimulating hormone (TSH) were studied. The glass tube was a plain glass tube, and the plastic tube was a serum separator tube with gel. The plastic vacuum tubes used in this study were the SST II plus tube (Becton Dickinson, Franklin Lakes, USA) and the Vacuette tube (Greiner Bio-One, Kremsmunster, Austria). An IMMULITE 2000 analyzer (Diagnostic Products Corporation, Los Angeles, USA) was used to measure the total T3, total T4 and TSH. RESULTS: Comparisons of the measured values within 1 hour of blood collection in the plastic tube with that in the glass tube are as follows. There was no difference between the Vacuette tube and the glass tube for the three tests, while there was statistically significant difference between the SST II plus tube and the glass tube for the total T3 and total T4. CONCLUSIONS: It might need more cautious interpretation of the results by a solid-phase, competitive chemiluminescent enzyme immunoassay, when the plastic vacuum tube is used as a blood collection tube instead of the glass tube.