When the body breaks down the thyroid hormone T4, the iodine atoms are released slowly as iodide, and this delay can only be explained by the presence of intermediate hormone fragments, such as diiodothyronines, that form temporarily before breaking down completely.
Evidence from Studies
No evidence studies found yet.
What Would Prove This
Per GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this claim, ordered from strongest to weakest.
A systematic review could confirm whether the inclusion of diiodothyronine kinetics improves model fit across multiple independent tracer studies in healthy humans.
A systematic review and meta-analysis of all published kinetic models using T4 tracer studies (n≥15), comparing model fit statistics (AIC, R²) with and without diiodothyronine submodels to determine if their inclusion consistently improves accuracy.
An RCT cannot ethically or practically test the existence of transient intermediates in healthy individuals.
Not applicable—no ethical or feasible RCT design can directly test transient metabolic intermediates in healthy humans without invasive tracer manipulation.
A cohort could correlate diiodothyronine levels with T4 degradation rates, but these intermediates are not routinely measurable.
A prospective cohort of 200 healthy adults undergoing serial plasma sampling after [131I]T4 injection, with advanced chromatography to quantify transient diiodothyronine levels and correlate their appearance timing with iodide release.
A cross-sectional study could measure diiodothyronine concentrations in healthy individuals, but cannot resolve kinetic timing.
A cross-sectional study measuring plasma concentrations of 3,3'-T2, 3,5-T2, and other diiodothyronines in 150 healthy adults using high-resolution mass spectrometry, alongside T4 and iodide levels.
A case report could describe abnormal diiodothyronine accumulation in a patient with deiodinase deficiency, supporting their role as intermediates.
A case series of 5–8 patients with known deiodinase mutations, undergoing [131I]T4 tracer studies to observe abnormal accumulation of diiodothyronines and delayed iodide release compared to healthy controls.