Radiologia Brasileira - Publicação Científica Oficial do Colégio Brasileiro de Radiologia

Maria Cristina Chammas

Director for the Service of Ultrasound at Instituto de Radiologia do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, São Paulo, SP, Brazil. E-mail: mcchamas@ hotmail.com

Undoubtedly, amongst the thyroid gland diffuse diseases (diffuse goiter, Graves' disease, viral thyroiditis, autoimmune thyroiditis, etc.), lymphocytic thyroiditis (or Hashimoto's thyroiditis) is the most frequent, with higher incidence in women than in men, in a 9:1 ratio.

Several reasons have been attributed to the increase in thediagnosis of lymphocytic thyroiditis in the Brazilian femalepopulation.

One of these reasons, frequently discussed in the media, isthe excessive consumption of iodine-enriched salt (by law, extraiodine is added to salt), which for three or four years, may leadgenetically predisposed individuals to develop autoimmunethyroiditis. Besides, it is known that in tropical countries likeBrazil, the food uses to be high in salt. Considering this fact,the mandatory addition of iodine to salt (at a range of 40 to 100mg of iodine/kg of salt) was changed by Anvisa in 2003, to 20–60mg of iodine/kg of salt.

At the same time, an increasing demand for thyroid ultrasound(US) has been observed, so presently this method is included inthe routine screening for women. Additionally, ultrasoundequipment technological developments allow the identification ofminimal alterations of the thyroid gland.

As a result of the above mentioned factors, subtle alterationscan be found in the thyroid parenchyma, allowing the earlydiagnosis of diseases. However, misinterpretation of thesefindings may generate problems in the management of the disease,besides anxiety for the patient.

In the case of autoimmune thyroiditis, ultrasound may or notdemonstrate textural alterations. If so, from a subtle, diffusetextural alteration to a marked hypoechogenicity of the gland maybe evidenced, remembering that in 90% of times hypoechogenicglands result from autoimmune diseases, the greatest part of themrepresented by thyroiditis or Graves' disease. In cases of markedhypoechogenicity and hyperechogenic fibrotic tissue crossing theparenchyma, there is no doubt, it is a classic case ofthyroiditis and the diagnosis is concluded.

However, heterogeneous textural patterns are included in thisarray of sonographic alterations. I am referring to the thyroidparenchyma intermixed with focal alterations mimicking nodules, acommon finding in thyroiditis and that frequently leave us indoubt about its interpretation. Some tips to avoid this pitfallare: observe whether these hypoechogenic areas are presentthroughout the whole parenchyma (following a symmetrical patternof distribution); observe whether there is no prominenthypoechogenic area; observe whether there is not a noduleconformation in the several cuts performed on a same dubiouslesion. These findings are compatible with pseudonodular areas,false nodules meaning lymphocytic infiltrate typical ofthyroiditis.

If the doubt persists, another resource can be utilized: thecolor Doppler mapping. In cases where a true nodule is notcharacterized, the color Doppler mapping demonstrates the absentdeviation of vessels in that region of the thyroidparenchyma.

In case the study indicates a non-characterization of a truenodule, it is recommended that these hypoechogenic alterationsare described as an "ill-defined hypoechogenic area" or a similarterminology, avoiding the term "nodule". This prudent measure isimportant since a physician faced with a sonographic reportasserting the existence of a nodule, will be induced to proceedwith the diagnostic investigation.

On the contrary, if any nodule is identified in the glandaffected by thyroiditis, it must be more deeply studied by meansof fine needle aspiration biopsy. Several authors havedemonstrated an increased risk for carcinoma in patients withthyroiditis because of the intense mitosis and cellularproliferation in this gland stimulated by TSH (thyroidstimulating hormone) and due to the lymphocytic infiltrate(leading to the cellular death).

Additionally, the follow-up of patients with thyroiditis isimportant, considering that, as already proven, thyroid lymphoma,although rare, may be preceded by chronic thyroiditis.

US Color Doppler can provide valuable information. However,the care in the performance of this examination should be doubledin relation ultrasound B-mode scan. The technique, the transducerand equipment set-up are essential factors for the accuracy ofthyroid US. Equipment sensitivity and knowledge on the technique(particularly the pressure of the transducer on the skin thatmust be minimal) should be added to these variables in the caseof the color Doppler method. According to the RadiologicalSociety of North America (RSNA) recommendations, the colorDoppler mapping must be performed with a sensitive equipment andthe operator must be especially trained for this purpose,otherwise it would rather not to utilize this method since thebenefit to the patient could be dubious⁽¹⁾.

At color Doppler, thyroiditis usually presents morevascularized than the typical pattern of the thyroid gland. Froma mild to quite accentuated increase in vascularization may beobserved. This data is particularly interesting in the evaluationof glands with minimal or dubious textural alteration, allowingthe definition of the diagnosis according to the degree of thethyroid gland vascularization.

Another information provided by the US color Doppler is thespectral analysis of the thyroid arteries which in thyroiditispresents blood flow velocities within normality limits (notsurpassing 40 cm/s); this does not occur in the Graves' disease(usually the blood flow velocity is > 50 cm/s in non-treatedcases or in those with inadequate response to treatment),allowing the differential diagnosis between these two diseaseswhen B-mode scan and/or color Doppler mapping present overlappingfindings.

At the study completion, it is convenient to perform cervicallymph nodes scan. In cases of thyroiditis, round and hypoechoiclevel VI lymph nodes have been observed. This finding isespecially significant, since level VI lymph nodes arespecifically responsible for the drainage of tumors of larynx,thyroid and other structures. So, while the identification oflymph nodes in this region is reported, it is necessary todescribe their characteristics, defining their pattern (suspector reactional).

Reference

1. Management of thyroid nodules detected at US: Society of Radiologists in US Cosensus Conference Statement. Radiology 2005;237:94–800.