abnormal thyroid gland morphology abnormal thyroid gland isthmus morphology MP:0013162 MPheno.ontology abnormal isthmus glandulae thyroideae morphology abnormal thyroid isthmus morphology any structural anomaly of the narrow median bridge that joins together the lower thirds of the two lateral (right and left) lobes of the thyroid gland and usually covers the second and the third tracheal rings; the thyroid isthmus is variable in presence and size, can change shape and size, and can encompass a cranially extending pyramid lobe (lobus pyramidalis or processus pyramidalis), remnant of the thyroglossal duct