GO:0046146

The chemical reactions and pathways involving tetrahydrobiopterin, the reduced form of biopterin (2-amino-4-hydroxy-6-(1,2-dihydroxypropyl)-pteridine). It functions as a hydroxylation coenzyme, e.g. in the conversion of phenylalanine to tyrosine. [CHEBI:15372, ISBN:0198506732]