Catalyzes the formation of L-carnitine from gamma-butyrobetaine. 2-oxoglutarate + 4-(trimethylamino)butanoate + O2 = carnitine + CO2 + succinate Binds 1 Fe(2+) ion per subunit. Amine and polyamine biosynthesis; carnitine biosynthesis. Highly expressed in kidney; moderately expressed in liver; very low expression in brain. Belongs to the gamma-BBH/TMLD family.