T4 + T3 combination therapy: any progress?

Area of Science:

• Endocrinology
• Thyroidology
• Pharmacology

Background:

• High dissatisfaction with levothyroxine (L-T4) monotherapy for hypothyroidism persists.
• Patients on L-T4 often report persistent symptoms like fatigue, weight gain, and cognitive dysfunction.
• Low serum T3 levels are common in L-T4 treated patients, necessitating TSH-suppressive doses for normalization.

Purpose of the Study:

• To review the understanding of T4+T3 combination therapy for hypothyroidism 7 years after 2012 guidelines.
• To investigate the reasons for persistent symptoms in patients treated with L-T4 monotherapy.
• To assess the current evidence and future directions for combination therapy.

Main Methods:

• Literature review of studies published between 2012 and 2019 on T4+T3 combination therapy.
• Analysis of thyroid hormone metabolism and peripheral tissue function during T4 monotherapy.
• Examination of genetic factors, such as DIO2 polymorphisms, potentially affecting T3 conversion.

Main Results:

• L-T4 monotherapy is associated with suboptimal T3 levels and variable tissue thyroid status.
• A normal serum TSH does not guarantee a euthyroid state due to differential T4 metabolism.
• A specific SNP in DIO2 may contribute to local hypothyroidism in the brain, explaining persistent symptoms.

Conclusions:

• The 2012 European Thyroid Association (ETA) guidelines for T4+T3 combination therapy remain relevant.
• Individualized, experimental T4+T3 treatment may be considered for selected patients with persistent symptoms.
• Well-designed randomized clinical trials are crucial for advancing treatment strategies and understanding the complexities of thyroid hormone therapy.