The relevance of T3 in the management of hypothyroidism.

Levothyroxine monotherapy has been the standard of care for treatment of hypothyroidism for more than 40 years. However, patients treated with levothyroxine have relatively lower serum tri-iodothyronine (T3) concentrations than the general population, and symptoms of hypothyroidism persist for some patients despite normalisation of thyroid-stimulating hormone (TSH) concentrations. The understanding that maintenance of normal T3 concentrations is the priority for the thyroid axis has redirected the clinical focus to serum T3 concentrations in patients with hypothyroidism. This Personal View explores whether it is currently feasible to identify patients who could be considered for liothyronine supplementation in combination with levothyroxine. Genetic profiling stands out as a potential future tool to identify patients who do not respond well to levothyroxine due to suboptimal peripheral thyroxine (T4) activation. Moreover, new slow-release liothyronine preparations are being developed to be trialled in these symptomatic patients, in an attempt to restore T3 concentrations and provide conclusive results for the use of T4 plus T3 combination therapy.