For a better assessment of thyroid function, each laboratory should establish its own reference intervals (RI). In Lebanon, no previous study has been conducted to establish the reference values for thyroid function tests. This cross-sectional study included 301 volunteers aged between 18 and 65 years (65.8% women, 34.2% men), free from any thyroid pathology and any condition that could affect thyroid function tests. The reference intervals of thyroid-stimulating hormone (TSH), free T4 (FT4), free T3 (FT3), and total T3 (TT3) were measured on the Roche Cobas e411 machine. 39 subjects tested positive for thyroid peroxidase antibodies (TPOAb) and/or thyroglobulin antibodies (TGAb), were excluded from the study. There was an increase in the 2.5th percentile for the TSH compared to the value provided by the manufacturer (from 0.27 to 0.53 mlU/L) as well as a decrease in the 97.5th percentile for the FT4, TT3, and FT3 (respectively, from 22 to 19.78 pmol/L for FT4, from 3.1 to 2.71 nmol/L for TT3, and from 6.80 to 6.10 pmol/L for FT3). Higher TT3 and FT3 values were noted in men compared to women (respectively, p = 0.068 and p < 0.0001). An age decrease in FT4 and FT3 was also observed (respectively, p = 0.045 and p < 0.0001). The established RI of thyroid function tests for the Lebanese population were significantly different from the values recommended by the manufacturer (Roche Diagnostics). Changing our RI values will allow a more accurate diagnosis of thyroid dysfunction.