Stem cells are critical for the regeneration and homeostasis of adult tissues. Thyroid hormone (TH), whose intracellular concentration is increased by Type 2 deiodinases (D2), is implicated in multiple cell functions. While reduced TH levels play a role in muscle stem cell activation, nothing in known about the role of TH in quiescence. Here we show that D2 is specifically expressed and marks quiescent stem cells in muscle and skin and increases intracellular TH. Acute D2-depletion in quiescent muscle stem cells triggers their spontaneous transition from G 0 into a G Alert state. Upon muscle injury, D2-depletion increases the proliferative potential of activated precursor cells but impairs self-renewal of progenitors returning to quiescence. Genetic D2-depletion leads, in the long run or upon multiple injuries, to depletion of the stem cell pool and regenerative failure. Mechanistically, D2-produced TH sustains Notch signalling by directly promoting the expression of Notch receptors and their canonical target genes. In normal and pathological settings, transient drug-induced D2 blocking accelerates muscle regeneration and skin wound healing. In conclusion, a D2-induced increased in intracellular TH concentration is critical in maintaining stem cell quiescence and in regulating self-renewal. In this context, tissue-specific manipulation of TH may be an innovative therapeutic tool in the field of regenerative medicine.