Thyronines and thyronamines: dual modulators of synaptic plasticity, cognitive decline, and geriatric depression in the aging brain.

The aging brain induces cognitive deterioration and geriatric depression via synaptic dysfunction, neuroinflammation, and bioenergetic failure. Recent evidence identifies thyroid hormones (thyronines: T3, T4) and their decarboxylated derivatives (thyronamines, especially 3-iodothyronamine, T1AM) as essential endogenous regulators of these processes. Thyronines influence both genomic and non-genomic pathways in glutamatergic, GABAergic, cholinergic, and monoaminergic signaling, thereby modulating learning and memory. With aging, diminished expression of thyroid hormone transporters (MCT8, OATP1C1) and compromised astrocytic conversion of T4 to T3 result in brain-specific hypothyroidism, facilitating amyloid-β accumulation, tau hyperphosphorylation, and hippocampal dysfunction. In contrast, T1AM stimulates trace amine-associated receptor 1 (TAAR1), promoting ERK1/2 phosphorylation, triggering autophagy through mTOR inhibition, and diminishing Aβ neurotoxicity in preclinical models. In geriatric depression, thyronines regulate monoaminergic transmission, whereas T1AM metabolites affect histaminergic pathways. Both hypothyroidism and hyperthyroidism significantly elevate the risk of Alzheimer's disease via distinct mechanisms: deficiency hinders glucose transport and amyloid-beta clearance, whereas excess disrupts mitochondrial respiration and activates stress kinases. The U-shaped dose-response relationship highlights the necessity for precise therapeutics. Selective modulators of thyronine and thyronamine receptors with improved brain penetration are promising approaches for addressing age-related cognitive decline and late-life depression.