Hypothyroidism is a common endocrine disorder affecting 3-15% of the adult population in subclinical form and 0.3-0.8% as overt disease. The mainstay of treatment is replacement monotherapy with levothyroxine (LT4). Currently several oral LT4 formulations including tablets, softgel capsules, and liquid formulations are available. Liquid LT4 is manufactured as LT4 solution in 85% glycerol and 96% ethanol and as LT4 solution in purified water and glycerol. The latest formulation, Tirosint SOL, gained FDA approval in 2017. To evaluate the clinical utility of liquid LT4 we reviewed the literature using three databases: PubMed/MEDLINE, Scopus, and Embase and found 405 articles among which 23 prospective and two retrospective studies were further evaluated. Finally, several case reports on rare clinical conditions were discussed. Our review demonstrated that liquid LT4 was more effective than tablet formulation in patients with malabsorption caused by interfering diseases, drugs, and bariatric surgery. The better pharmacokinetics of liquid LT4 was also confirmed in subjects without malabsorption: patients on replacement or suppressive therapy, who switched from tablet to liquid formulation in equivalent dose, gained better hormonal control, and required less frequent TSH measurements. The drug also appeared effective and easy to handle in patients fed by enteric tube. Liquid LT4 appeared equally effective whenever taken before or during breakfast. The analysis of the drug utility in particular populations including newborns, pregnant women, and the elderly confirmed the high value and safety of liquid LT4. However, in neonates the higher incidence of TSH suppression on liquid in comparison to tablet LT4 therapy was noted, and particular attention to avoid over-treatment must be paid. Concluding: the literature review revealed that liquid LT4 is especially advantageous in patients with malabsorption and the critically ill, but it seems also very promising in common therapy. The lack of alcohol content in the new formulation makes Tirosint SOL especially attractive.
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