**2. Radiopharmaceuticals principles**

Radiopharmaceuticals are substances that contain one or more radioisotopes (radionuclides). They are nonencapsulated sources of artificial ionizing radiation, which are used for both diagnostic and therapeutic medical applications in the field of nuclear medicine.

It is important to mention certain general characteristics, such as the type of radioactive emission (gamma photons, alpha or beta particles and mixed, with emission of gamma photons and charged particles), the emission energy measured in KeV and the physical half-life (T ½ f), that help us to know and properly choose the type of radiotracer that we should use at all times. Radioiodine isotopes and 99mTc-pertechnetate (TCO4-) are the most commonly used radiopharmaceuticals for thyroid imaging.

99mTc-pertechnetate is used worldwide to study the thyroid function because of its advantages, such as a short retention in the gland due to half-life (6 h) and no beta-radiation, thus providing low dosimetry to the thyroid gland and the rest of the body. Its gamma photon of 140 keV is ideal for imaging using scintillation cameras, really cost effective and it can be done fast (readily available), safe and no side effects [5, 6]. A disadvantage is that 99mTc is only trapped and not organified in the follicles [7].

Iodine-123 (123I) is both trapped and organified by the thyroid gland, it has a relatively short half-life of 13.6 h, a gamma photon suitable for imaging using conventional scintillation cameras (159 keV) and no beta-radiation [5, 7]. Therefore, it is considered the ideal agent for thyroid imaging. However, the reality is that its availability is limited and costly due to its expensive and complex production in a cyclotron. As the information is mostly the same as that obtained by 99mTc-pertechnetate scintigraphy, specific indications include evaluation of organification defects [5–7].

Iodine-131 (<sup>131</sup>I) was frequently used in the past in thyroid diagnosis imaging because of both gamma emission (364 keV) and beta particle emission [7]. Its special characteristics of energy emission, its long half-life (approximately 8.1 days) and high radiation doses to the gland (1–3 rad/mCi) makes 131-Iodine less satisfactory for thyroid imaging (poor quality images are produced) [5]. Currently, 131-Iodine is a radiopharmaceutical used mainly for metabolic therapy in benign thyroid disorders (thyroid hyperfunction) and ablation of tumor remnants of differentiated thyroid carcinomas, in addition to the staging and follow-up of patients with such tumors (using a lower dose of 131I than in the ablation of possible thyroid remnants) [6].
