The effects of combination treatment with LT4 and LT3 on diastolic functions and atrial conduction time in LT4-treated women with low T3: a short term follow-up study

Purpose To investigate the effects of combination treatment with levothyroxine (LT4) and l- triiodothyronine (LT3) on left atrial volume (LAV), diastolic functions, and atrial electro-mechanical delays in LT4-treated women with low triiodothyronine (T3) levels. Methods This prospective study consisted of 47 female patients between 18 and 65 years old treated at an Endocrinology and Metabolism outpatient clinic between February and April 2022 due to primary hypothyroidism. The study included patients with persistently low T3 levels in at least three measurements, despite LT4 treatment (1.6-1.8 mcg/kg/m(2)) for 23.13 +/- 6.28 months with normal thyrotropin (TSH) and free tetraiodothyronine (fT4) levels. The combination therapy dose was as follows: the fixed LT4 dose (25 mcg) was removed from patients' usual LT4 treatment [100 mcg (min-max, 75-150)], and a fixed LT3 dose (12.5 mcg) was added. Biochemical samples were taken, and an echocardiographic assessment was performed for patients upon their first admission, and after 195.512.8 days of receiving LT3 (12.5 mcg) treatment. Results There was a statistically significant reduction at left ventricle (LV) end-systolic diameter (27.69 +/- 3.14, 27.13 +/- 2.89, p = 0.035), left atrial (LA) maximum volume (14.73 +/- 3.22, 13.94 +/- 3.15, p = 0.009), LA minimum volume (7.84 +/- 2.45, 6.84 +/- 2.30, p < 0.001), LA vertical diameter (44.08 +/- 6.92, 34.60 +/- 4.31, <0.001), LA horizontal diameter (45.65 +/- 6.88, 33.43 +/- 4.51, p < 0.001), LAVI (50.73 +/- 18.62, 41.0 +/- 13.02, p < 0.001), total conduction time (103.69 +/- 12.70, 79.82 +/- 18.40, p < 0.001) after LT3 replacement (respectively pre-post- treatment and p value). Conclusion In conclusion, the findings of this study suggest that the addition of LT3 to LT4 treatment may lead to improvements in LAVI and atrial conduction times in patients with low T3. However, further research with larger patient groups and exploration of different LT4 + LT3 dose combinations is needed to better understand the effects of combined hypothyroidism treatment on cardiac functions.