isthmus of thyroid gland
A 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.