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BACKGROUND: Combination therapy with levothyroxine (L-T4) and slow-release T3 (SRT3) in the treatment of hypothyroidism results in a normal triiodothyronine/thyroxine (T3/T4) ratio above that of L-T4 monotherapy. No clinical study has been reported on SRT3 monotherapy for hypothyroidism. METHODS: This study was conducted in two parts. In the first part, 20 patients with primary hypothyroidism and serum thyrotropin (TSH) >30 mU/L were randomized into three groups receiving 1.6 μg/kg L-T4, equivalent doses of SRT3 or L-T3 of 0.55 μg/kg for 4 weeks. Their fasting serum-free T4 (fT4), T3, and TSH were measured weekly before taking medication for up to 4 weeks. In the second part, in 9 hypothyroid patients on L-T4 therapy and normal serum TSH, LT4 therapy was discontinued, and a once-daily dose of SRT3 of 0.55 μg/kg was replaced. Serum fT4, T3, and TSH were measured weekly. RESULTS: In part one, in patients treated with L-T3 and L-T4, serum TSH decreased to normal values after 4 weeks of intervention. In 7 patients on SRT3, serum T3 increased from 47 ± 12 at baseline to 110 ± 16 ng/dL, and serum TSH decreased from 60 ± 11 at baseline to 24 ± 10 and 26 ± 7 mU/L, respectively, at 14 and 21 days after intervention. At the end of 28 days, mean serum T3 was 110 ± 16, 168 ± 74, and 96 ± 18 ng/dL in SRT3, L-T3, and L-T4 groups, respectively (p < 0.001). In part two, serum fT4 decreased from 1.43 ± 0.7 to 0.41 ± 0.14 ng/dl, and serum T3 increased from 86 ± 21 to 113 ± 27 ng/dL by 21 days. Mean serum TSH remained normal until 14 days but increased to 15.1 ± 7.6 mU/L at 21 days. In the end, mean serum fT4, T3, and TSH were 0.35 ± 0.17 ng/dl, 77.4 ± 8.9 ng/dL, and 35 ± 11 mU/L, respectively. CONCLUSION: In patients with primary hypothyroidism, SRT3 monotherapy with an equivalent dose to L-T4 maintained normal serum T3 but could not sustain normal serum TSH concentration. CLINICAL TRIAL REGISTRATION NUMBER: IRCT20100922004794N12.