The metathalamus was a composite structure of the thalamus, consisting of the medial geniculate nucleus and the lateral geniculate nucleus.
In 2366, when Ambassador Sarek was suspected of suffering from suffering from Bendii Syndrome, Dr. Beverly Crusher suggested confirming it by growing a culture from the tissue of his metathalamus. Unfortunately, it would have taken several days for the results to have come through. (TNG: "Sarek")