**4. Diagnostic methods in the evaluation of hyperthyroidism**

The normal thyroid gland and anatomic variants can be visualized by numerous imaging modalities including scintigraphy, ultrasound and computed tomography. Although magnetic resonance imaging (MRI) is capable of providing excellent anatomic detail of the thyroid gland using proton density imaging, it is not usually used in routine clinical practice [7].

Thyroid ultrasonography utilizes reflected sound waves that allow to identify and evaluate gland size, location, the presence of nodules and to differentiate between cystic and solid lesions [7].

There are certain echographic characteristics such as echogenic behavior of the thyroid nodule (as purely cystic or hyperechoic nodules), good demarcation and external vascularization suggesting signs of benign course (e.g., a thyroid adenoma). On the contrary, solid hypoechogenic nodules, irregular borders, internal vascularization, presence of microcalcifications or recent increase in size on follow-up are associated with malignancy [10].

Ultrasound is, therefore, an image modality from which morphological information of the thyroid gland is obtained. On the other hand, thyroid scintigraphy is a functional imaging test that visualizes the distribution of active thyroid tissue and is used as a complementary test for definite diagnosis [6, 10].

*American Thyroid Association (ATA)* and other guidelines have published their recommendations suggesting that thyroid scintigraphy is useful in the assessment of diffuse goiter *(Graves' disease)*, the simple thyroid nodule and multinodular goiter, acute or chronic local inflammation (thyroiditis), suspected ectopic thyroid (such as lingual thyroid), the study of cervical embryonic development anomalies of thyroid origin (thyroglossal cysts) to evaluate the extent of retroesternal goiter and patients undergoing treatment with radioactive iodine, which is important to know about the anatomical distribution, active thyroid tissue information and select the appropriate therapeutic dose/activity of 131-Iodine [6].

Regarding the evaluation of thyroid nodules by thyroid scintigraphy, it plays an important role in the identification of the functional state of the nodule. Non-functioning thyroid nodules do not present radiotracer uptake ("cold nodules") and present a higher risk of malignancy, while functioning thyroid nodules have tracer uptake ("hot nodules") and are usually benign nodules [10].

Laboratory tests play a fundamental role in the initial diagnosis and follow-up of thyroid hyperfunction, in the assessment of possible autoimmunity associated with thyrotoxicosis, in the control and adjustment of adequate pharmacological dose to each patient, as well as in the detection of pharmacological response (drug resistance) or clinical relapse.

In the presence of typical signs and symptoms, a TSH suppressed with excess thyroid hormone production—thyroxine (T4), free thyroxine (FT4) and/or triiodothyronine (T3)—indicates clinical-analytic findings of hyperthyroidism. In the case of *Graves' disease*, these hormonal alterations are attributed to the presence of thyroid stimulating antibodies (TSHR-Ab), specifically thyroid stimulating immunoglobulins (TSI) [2, 11]. This existence of antibodies in the bloodstream explains the autoimmune and genetic component of this syndrome, as well as its relationship with other autoimmune entities.
