Background: Triiodothyronine (T3), the most active thyroid hormone, stimulates energy expenditure, but is not a viable weight loss therapy due to potential adverse cardiovascular effects. T3 binds to two receptors: TRα predominates in cardiac and skeletal muscle and TRβ in the central nervous system. Mutations in TRβ provide an opportunity to assess the effect of selective TRβ receptor stimulation as a potential therapeutic target for weight loss, while avoiding adverse cardiovascular effects.
Methods: Six healthy female controls (age 36±3 years, fT3=4.3±0.2 pmol/L) and two female patients with mutations in TRβ (age 25 and 40 years, fT3=7.8 and 10.0 pmol/L) were studied before and after two days of T3 (100 µg/day). Resting energy expenditure (REE) and diet-induced thermogenesis (DIT) were assessed by indirect calorimetry performed before and after a mixed meal. Pulse, blood pressure (BP) and low-density lipoprotein cholesterol (LDL-C) were measured and brown adipose tissue (BAT) activity was assessed by infrared thermography.
Results: Following T3 treatment, mean ΔfT3 was +15.5±1.6 pmol/L in controls and +15.3±3.3 pmol/L in patients with TRβ mutations. In controls, T3 increased REE (Δ+2.3±0.7 kcal/kg LBM/day, p=0.02) and pulse (Δ+ 6.5±0.8 bpm, p<0.001) and tended to reduce LDL-C (Δ-0.5±0.2 mmol/L, p=0.09). T3 did not affect DIT (p=0.98), BAT activity (p=0.18) or systolic BP (p=0.81). Patients with TRβ mutations had higher REE and pulse, but not LDL-C, at baseline, but an attenuated change in REE, pulse and LDL-C after T3 (Figure). DIT, BAT activity and BP did not differ between groups.
Conclusions: T3 effects on REE and heart rate are mediated by both TRα and TRβ, while change in LDL-C is via TRβ. Selective TRβ stimulation will likely increase REE with consequent weight loss and reduce LDL-C. However, although the effect on heart rate is predicted to be less than T3, an increase in pulse is expected.