Monocarboxylate transporter 8 acts as a specific cell membrane transporter for thyroxine and especially triiodothyronine into target cells. It is expressed in brain neurons and in many other tissues. The monocarboxylate transporter 8 gene resides on chromosome Xq13.2. An 11-month-old male infant was referred because of severe hypotonia from early life and global developmental delay. Thyroid-function tests showed normal thyrotropin levels and the characteristic for the disorder, including high serum triiodothyronine and low thyroxine concentrations. Molecular analysis of the monocarboxylate transporter 8 gene showed that the patient was hemizygous for a novel missense mutation P537L. This case highlights the importance of determining thyroid hormone levels, especially triiodothyronine, in infants with severe neonatal hypotonia.