Thyroid function is clinically evaluated by determination of circulating concentrations of TSH and fT4. However, a tissue-specific effector substrate of thyroid function is lacking. Energy-rich phosphorus-containing metabolites (PM) and phospholipids (PL) might be affected by thyroid hormone action and can be non-invasively measured by 31P nuclear magnetic resonance spectroscopy (NMRS).Measuring the actions of peripheral thyroid hormones on PM and PL tissue concentrations.A longitudinal, prospective pilot study was performed.Nine patients with hyperthyroidism (HYPER) and four patients with hypothyroidism (HYPO) were studied at baseline and three months after treatment.High field 1H/ 31P NMRS was used to assess profiles of PM, PL and flux through oxidative phosphorylase in liver and skeletal muscle, as well as ectopic tissue lipid content.The concentrations of total skeletal muscle (m-) and hepatic (h-) phosphodiesters (PDE) and of one of its constituents, glycerophosphocholine (GPC), were lower in HYPER compared to HYPO (m-PDE: 1.4±0.4mM vs. 7.4±3.5mM, p=0.003; m-GPC: 0.9±0.3mM vs. 6.7±3.5mM, p=0.003; h-PDE: 4.4±1.4mM vs. 9.9±3.9mM, p=0.012; h-GPC: 2.2±1.0mM vs. 5.1±2.4mM, p=0.024). Both h-GPC (rho=-0.692, p=0.018) and h-GPE (rho=-0.633, p=0.036) correlated negatively with thyroxine (fT4). In muscle tissue, a strong negative association between m-GPC and fT4 (rho=-0.754, p=0.003) was observed.Thyroxine is closely negatively associated with the PDE concentrations in liver and skeletal muscle. Normalization of thyroid dysfunction resulted in a decline of PDE in hypothyroidism and an increase in hyperthyroidism. Thus, PDE might be a sensitive tool to estimate tissue-specific peripheral thyroid hormone action.