Abstract: The use of a monocarboxylate transporter protein or a functional part, derivative and/or analogue thereof for altering transport of a thyroid hormone or a functional part, derivative and/or analogue thereof across a membrane. The monocarboxylate transporter protein preferably comprises MCT-8. An isolated molecule capable of specifically binding at least part of an MCT protein, at least part of a ligand of an MCT protein, or a nucleic acid encoding the MCT and/or ligand, is also herewith provided. Regulation of the bioavailability of thyroid hormone in a tissue enables interfering with diseases. The invention also provides pharmaceutical compositions comprising a compound capable of binding the MCT protein or capable of influencing the binding or transporting of a ligand of the MCT protein.