An isthmus is a narrow connection between two larger body parts. In sonography, the term is most frequently used to describe the thin bridge of thyroid tissue connecting the right and left lobes of the thyroid gland, which crosses the anterior surface of the trachea. On ultrasound, the thyroid isthmus is measured in the transverse plane and normally measures less than 3 to 5 millimeters in thickness. An abnormally thickened isthmus can indicate diffuse thyroid disease, and nodules can develop within the isthmus just as they can in the thyroid lobes. The term is also used for the narrow segment of the fallopian tube near its junction with the uterus and for the connecting tissue in a horseshoe kidney.
Sonography Term