Treatment of Primary Hypothyroidism by Slow-Release Liothyronine Monotherapy.

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 with SRT3 monotherapy for hypothyroidism.

Methods: This study was conducted in two parts. In part one, 20 patients with primary hypothyroidism and serum thyrotropin (TSH) >30 mU/L were randomized into 3 groups receiving 1.6 μg/kg L-T4, equivalent doses of SRT3 or L-T3 of 0.55 μg/kg for 4 weeks and fasting serum free T4 (fT4), T3 and TSH were measured weekly, before taking medication, up to 4 weeks. In part two, in 9 hypothyroid patients on L-T4 therapy and normal serum TSH, L-T4 therapy was discontinued and a once daily dose of SRT3 0.55 μg/kg was replaced. Serum fT4, T3 and TSH were measured weekly.

Results: 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. Part Two: serum fT4 decreased from 1.43±0.7 to 0.41±0.14 ng/dLand serum T3 increased from 86±21 to 113±27 ng/dL by 21 days. Mean serum TSH remained in the normal range until 14 days but increased to 15.1±7.6 mU/L at 21 days. At 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.

Conclusion: In patients with primary hypothyroidism SRT3 monotherapy with an equivalent dose to L-T4 maintains normal serum T3, but is unable to sustain normal serum TSH concentration.