In man carnitine is synthesized from proteic trimethyllysine in liver, brain and kidney. Muscles which contain approximately 98% of carnitine must take it up from the blood in an exchange process with endogenous deoxycarnitine, the immediate precursor of carnitine. Uneven organ distribution of the enzymes catalyzing carnitine synthesis further implies an interorgan transport of the intermediates. Assay of these intermediates in blood may assist causal definition of carnitine deficiency syndromes.

Besides catalyzing the transport of long-chain acyls in mitochondria, carnitine is necessary for the export of intramitochondrially produced short-chain acyls and for trapping and elimination of unphysiological acyls (benzoic, pivalic, valproic acids etc.). Unlike the corresponding acyl-CoA, carnitine esters are capable of diffusing across cellular membranes, and may be eliminated in urine, distributed in tissues or both. Assay of physiological and unphysiological carnitine esters in urine is necessary for the diagnosis of carnitine insufficiencies.