Background: Assessment of thyroid function is important during postpartum period. However, measurements of total thyroxine (TT4) and total triiodothyronine (TT3) levels by immunoassays are mostly inaccurate during pregnancy and postpartum period, as immunoassays are interfered by hormone-binding proteins. LC-MS/MS is known to be selective and specific for the analysis of endocrine hormones and can be utilized for the analysis of TT4 and TT3. The aim of this study is to evaluate the reliability of TT4 and TT3 results measured by LC-MS/MS during postpartum period.Materials and Methods: Serum samples (n=358) were obtained within postnatal 48hr from participants. TT4 and TT3 levels were measured by LC-MS/MS method (TT4(MS) and TT3(MS)). Levels of free T4 (FT4), free T3 (FT3) and thyroid-stimulating hormone (TSH) were measured by immunoassay methods (FT3(IA), FT4(IA,) and TSHIA).Results: Statistically significant correlation was found between TSHIA and TT4(MS) (r=-0.13 and P=0.01), whereas no correlation was detected between TSHIA and FT4(IA,) TT3(MS) or FT3(IA).Conclusions: Significant negative correlation between TT4(MS) and TSHIA, and lack of correlation between FT4(IA) and TSHIA indicates that TT4(MS) measured by LC-MS/MS could be a more reliable parameter in the diagnosis of thyroid deficiency during postpartum period.