Endocrine Abstracts

Background: It is well established that subclinical hypothyroidism (SCH) is associated with mild ventricular dysfunction and early cardiovascular disease (CVD), but it is unknown if there is an underlying defect in cardiac bio-energetic function.

Objective: To quantify the cardiac phosphocreatine/ATP (PCr/ATP) ratio in SCH, compare with healthy controls (HC) and to measure the effect of 6 months of thyroxine treatment.

Method: Cardiac energetic function (PCr/ATP ratio) was measured using phosphorus-31 magnetic resonance spectroscopy in subjects with SCH (TSH 4.0–10.0 mIU/l, normal free T₄) at baseline and after thyroxine therapy (1.6 cg/kg per day) and compared with age and gender matched HC. All subjects were free of any overt heart disease. 21 subjects with SCH and 17 HC were well matched for age (mean age 39.1 vs 43.3 years), sex (females 80 vs 80%), and mean free T₄ (13.4 vs 14.4 pmol/l) but differed in mean TSH (6.6 vs 2.1 mIU/l, P<0.001). A mean serum TSH of 2.0 mIU/l was achieved on treatment with thyroxine.

Results: At baseline PCr/ATP ratio in 21 SCH was lower than in HC (1.8±0.3 vs 2.1±0.2, P=0.001). After treatment (data analysed in 16 SCH) PCr/ATP ratio significantly improved (1.9±0.3 vs 1.7±0.2, P=0.004) to the level similar to HC (P=NS). Serum TSH was similar in HC and SCH post-treatment group (P=NS). The PCr/ATP ratio negatively correlated with serum TSH in all subjects (r=−0.37, P=0.026).

Discussion: To our knowledge, this is the first report demonstrating subnormal cardiac PCr/ATP ratio in SCH subjects and correction by thyroxine treatment. By comparison with studies of overt heart disease and PCr/ATP, our findings in SCH provide further confirmation for the presence of early cardiac impairment in our subjects, which is reversible with thyroxine therapy and provides a rationale for active management of SCH.

Declaration of funding

Bupa Foundation. The Medical Research Charity (Ref:22094167).