Chapter: Thyroid

Background Method

Thyroid

An overview of the pitfalls, cautions, and clear benefits of being able to examine the thyroid beyond palpation

Thyroid US raises some issues for primary care. If is fairly easy to perform, but requires experience to interpret and integrate the findings into clinical decision-making.

Fatal thyroid cancer is rare but thyroid nodules are common and often found incidentally when CT, MR, and neck US exams include the thyroid in the field. Of these accidentally discovered nodules, only about 10% turn out to be malignant. The great majority of the malignancies are small, low-grade papillary cancers which probably don’t need resection because there has been no detectable change in thyroid cancer death rate despite a large increase in thyroid cancer resections. All of this information indicates that routine screening for thyroid cancer is hard to support.

Yet, some patients come to clinic thinking that they have seen or felt something in their lower neck. Others come because their unrelated imaging study has shown something in the thyroid. Some clinic physicians can’t help themselves and examine the thyroid routinely as part of an annual physical. Competence at thyroid POCUS may avoid sending some patients for formal ultrasound studies.

Expertise in thyroid POCUS requires examination of more patients than just those who come with possible thyroid lumps. Quietly examining thyroids of patients on whom carotid and internal jugular exams are being performed is an acceptable learning approach. However, a “break the glass” approach is needed when an abnormality is seen that cannot be ignored. The finding is discussed with the primary physician, who makes the final decision about what to do.

Generally, only nodules > 1 cm need to be further evaluated. In addition, pure cysts of any size are never malignant. There are patient risk factors that increase the chance of thyroid cancer (family history, radiation as a child, genetic diseases with an increased risk of medullary carcinoma). A few ultrasound characteristics also increase the chance of malignancy, but these are imperfect: solid nodules that are hypoechoic, lobulated borders, capsular bulging, increased vascularity, and nodules that are taller than wide. However, the safest rule will be that any nodule > 1 cm that is not purely cystic probably needs a formal ultrasound and a TSH (to be sure it is not functional).


TECHNIQUE AND FINDINGS

Use the large linear L38 probe in clinic. For a specific thyroid US, use the Small Parts exam type. However, the venous setting used for carotid and internal jugular vein exams is acceptable for screening the size and composition of the thyroid. Most patients can keep their clothes on and be draped with washcloths for the exam. The patient should lie supine and attempt to extend the neck, but should not be

It makes sense to cover the area with gel before the exam. Start with the right side and place the probe transverse, indicator examiner left, above the clavicle as for a carotid/jugular vein exam. The probe is angled slightly medial to get a good image. The carotid is almost always at the lateral border of the thyroid. Optimize the depth and gain.

If the thyroid lobe is already visible just above the clavicle, be sure to fan down and look for a retrosternal thyroid. If no thyroid is visible right above the clavicle slide cephalad until the thyroid is seen just lateral to the trachea.

Size the right thyroid lobe before evaluating its composition. Quickly slide up and down in the transverse plane and identify the location where the thyroid appears the largest and FREEZE the image. Use calipers to measure the transverse width (to the beginning of the isthmus) and the anterior/posterior depth. A normal thyroid lobe is less than 2 cm in width and depth. Here is a normal right lobe of the thyroid with the width and depth measured.

After transverse sizing of the right lobe, slowly move up and down the lobe in the transverse plane again, looking at the parenchyma. It makes sense to briefly apply CD to the lobe, at least to be able to learn what normal looks like. Here is a clip from a patient having a carotid flow time and JVP performed, during which the right thyroid looked abnormal.

The largest nodule, with the hyperechoic walls, was about 1.5 cm in largest dimension. While it had cystic elements, it was not purely cystic. Cystic-appearing lesions should always have CD applied to be sure they are not vascular structures. The smaller nodule more lateral is under 1 cm with mixed density. If only this smaller nodule had been seen, it could have been followed in clinic for a size increase without doing a formal ultrasound. The larger nodule needed a formal ultrasound and biopsy.

After the transverse evaluation of the right lobe, rotate the probe to the parasagittal plane, indicator cephalad. Normal cephalad/cauda length is less than about 5 cm, but even with the long linear probe (L38 = 38 mm aperture), many normal thyroid lobes cannot fit on one screen. Fortunately, it is rare for a thyroid to be enlarged only because of its length so measuring just the width and depth is adequate in most patients. Here is a long axis measurement of a normal right thyroid lobe. There is some remaining lobe off the screen on the cephalad side.

Slide slowly medial to lateral (and up and down) through the lobe, looking for nodules. Here is the longitudinal view of the incidental thyroid nodule shown above. The depth adjustment could probably have been brought up one notch, but the examiners wanted the lesion in the middle of the sector for optimum focus. The complexity of the lesion is obvious.

To switch to the left lobe of the thyroid, cross over the trachea and evaluate the isthmus.  A normal isthmus should be less than 0.5 cm in depth. Here is a transverse isthmus in a normal, smaller adult that also showed both lobes of the thyroid.

The left thyroid lobe is examined in the same order as the right: transverse evaluation, followed by the longitudinal. The probe indicator stays examiner left, which shows the anatomy in correct position, with the carotid artery lateral to the thyroid. Here is a transverse view of a normal left thyroid lobe.

The following clip is from a patient who was having follow-up for an abnormal thyroid seven years before. This is a fairly large and complex lesion that was different from the seven years before, so this patient could not be considered stable and needed formal US and biopsy.

Finally, here is the longitudinal view of a normal left thyroid, again with some thyroid still above and below the field.