OBJECTIVE: The monocarboxylate transporter 8 (MCT8; SLC16A2) has a pivotal role 
in neuronal triiodothyronine (T(3)) uptake. Mutations of this transporter 
determine a distinct X-linked psychomotor retardation syndrome 
(Allan-Herndon-Dudley syndrome (AHDS)) that is attributed to disturbed thyroid 
hormone levels, especially elevated T(3) levels. We describe the genetic 
analysis of the MCT8 gene in a patient suspected for AHDS and the clinical and 
endocrine effects of L-thyroxine (LT(4)) or liothyronine (LT(3)) treatment 
intending to overcome the T(3) uptake resistance through alternative 
transporters.
METHODS: The six exons of the MCT8 gene were amplified individually by PCR. As 
multiple exons were missing, the length of the X-chromosomal deletion was 
determined by a dense SNP array, followed by PCR-based fine mapping to define 
the exact borders of the deleted segment. The clinical and endocrine data of the 
patient during 6.5 years of LT(4) treatment and two periods (3 months each) of 
low- and high-dose LT(3) were evaluated.
RESULTS: A partial deletion of the MCT8 gene (comprising five of six exons) was 
detected, confirming the suspected AHDS. MCT8 dysfunction was associated with 
partial resistance to T(3) at the hypothalamus and pituitary level, with normal 
responsiveness at the peripheral organs (liver and cardiovascular system). 
Thyroid hormone administration had no beneficial effect on the neurological 
status of the patient.
CONCLUSION: We identified a 70 kb deletion encompassing exons 2-6 of the MCT8 
gene in our AHDS patient. Both LT(4) and LT(3) administration had no therapeutic 
effect. Alternatively, treatment of AHDS patients with thyroid hormone analogs 
should be considered.