The US examination should begin with the patient in supine position,
with a pillow placed under their shoulders and hyperextend neck.
Ultrasound gel is applied over the thyroid area.
The transducer is directly placed on the skin over the thyroid gland and images of each lobe are obtained in transverse and longitudinal planes.
Craniocaudal and sagittal dimensions of both lobes are measured on the longitudinal image.
The transverse dimension is measured on the transverse image.
The thyroid gland volume is calculated using the formula: Volume = length x width x thickness x 0.532 (conversion factor),
where: length = craniocaudal dimension,
width = transverse dimension ,
thickness = sagittal dimension.
Normal volume : 6 to 15 cm3 ( 3-4 + 10-11 cm3 ).Fig. 1
The normal gland has characteristic echogenicity,
easily distinguishable from adjacent muscular structures.
The thyroid gland characteristic ecogenicity is due to its follicular structure: the interface among thyroid cells and colloid produces high acoustic impedancy,
causing high frequency sound waves to be reflected back to the probe. Fig. 2.
This results in greater ecogenicity than neck muscles and as intense (or even greater) ecogenicity than submandibular glands.
it is very important to classify the gland ultrasonographic aspect as isoechogenic,
hipoechogenic or hiperechogenic.
heterogeneous textural patterns are included in this array of sonographic alterations.
Some tips to avoid this pitfall are: observe whether these hypoechogenic areas are present throughout the whole parenchyma (following a symmetrical pattern of distribution); observe whether there is no prominent hypoechogenic area; observe whether there is not a nodule conformation in the several cuts performed on a same dubious lesion.
These findings are compatible with pseudonodular areas,
false nodules consisting of lymphocytic infiltrate typical of thyroiditis. Fig. 3 and Fig. 4
If doubt persists,
another resource can be used: color Doppler mapping.
In cases where a true nodule is not characterized,
color Doppler mapping demonstrates the absent deviation of vessels in that region of the thyroid parenchyma.
In case the study indicates a non-characterization of a true nodule,
it is recommended that these hypoechogenic alterations are described as “ill-defined hypoechogenic area” or a similar terminology,
avoiding the term “nodule”.
This prudent measure is important since a physician faced with a sonographic report asserting the existence of a nodule will be induced to proceed with the diagnostic investigation.
US color Doppler can provide valuable information.
extra care should be taken in performing color Doppler.
the transducer and equipment set-up are essential factors for the accuracy of thyroid US.
Equipment sensitivity and knowledge of the technique (particularly the transducer pressure on the skin must be minimal) should be added to these variables in the case of the color Doppler method.
It is usually observed in GD: diffusely increased vascularity (“thyroid inferno”) and elevated systolic peak velocity (SPV) (>50 cm/s).Fig. 5, Fig. 6 . In HT the most common findings are: normal or increased volume and vascularization (never as high as in GD) and moderately elevated SPV (30 cm/s < SPV < 50 cm/s).
In the case of autoimmune thyroiditis,
ultrasound may or not demonstrate textural alterations.
subtle to marked textural alteration of the gland may be identified.
It is important to note that 90% of hypoechogenic glands result from autoimmune diseases,
most of them HT or GD.
The classic pattern of thyroiditis is a marked gland hypoechogenicity with hyperechogenic fibrotic tissue crossing the parenchyma.
At the study completion,
it is convenient to perform cervical lymph nodes scan.
In cases of thyroiditis,
round and hypoechoic level VI lymph nodes are common.
This finding is especially significant,
since level VI lymph nodes are responsible for the drainage of tumors of larynx,
thyroid and other structures.
when lymph nodes in this region are reported,
it is necessary to describe their characteristics and define their pattern (suspect or inflammatory).