Primary carnitine deficiency (PCD) is a rare autosomal recessive disorder caused by loss of function mutations in the solute carrier family 22 member 5 (SLC22A5) gene that encodes a high-affinity sodium-ion-dependent organic cation transporter protein (OCTN2). Reduced carnitine transport results in diminished fatty acid oxidation in heart and skeletal muscle and carnitine wasting in urine. We present a case of PCD diagnosed in an adult female after a positive newborn screen (NBS) for PCD that was not confirmed on follow-up testing. The mother was referred for evaluation of persistent fatigue and possible hypothyroidism even though all measurements of thyroid-stimulating hormone were well within the range of 0.4 to 2.5 mIU/L expected for reproductive-age women. She was found to have unequivocally low levels of both total carnitine and carnitine esters, and genetic testing revealed compound heterozygosity for 2 SLC22A5 mutations. One mutation (c.34G>A [p.Gly12Ser]) is a known missense mutation with partial OCTN2 activity, but the other mutation (c.41G>A [p.Trp14Ter]) is previously unreported and results in a premature stop codon and truncated OCTN2. This case illustrates that some maternal inborn errors of metabolism can be identified by NBS and that maternal carnitine levels should be checked after a positive NBS test for PCD.