Serum T3 levels remain fairly constant in healthy individuals, but D2 and D3 can regulate tissue specific intracellular levels of T3 to maintain homeostasis since T3 and T4 levels may vary by organ. Deiodinases also provide spatial and temporal developmental control of thyroid hormone levels. D3 levels are highest early in development and decrease over time, while D2 levels are high at moments of significant metamorphic change in tissues. Thus D2 enables production of sufficient T3 at necessary time points while D3 may shield tissue from overexposure to T3.