**1. Introduction: thyroid anatomy and physiology**

Thyroid gland is the first organ to develop in human embryo. Its development begins 22 days after conception. The thyroid gland develops in the floor of the primitive foregut, between the first and second pharyngeal pouches from the endoderm. It descends to its habitual position, by the anterior neck to the level of the trachea, connecting to the tongue's base by the thyroglossal duct. The thyroglossal duct starts from the foramen caecum and normally involutes

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

throughout the development of the embryo when the thyroid occupies its final position in the neck, but sometimes becomes into a pyramidal lobe which is contiguous with the thyroid isthmus [1].

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].

Nuclear Medicine in the Assessment of Thyrotoxicosis Associated with Increased Thyroid…

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A thorough cervical examination is important. The palpation of the thyroid gland should be done with the patient sitting (never in supine position) and helping with swallowing movements. We must be careful in the search for possible goiters and their correlation with size (from small goiters, grade-I, to large goiters of endothoracic clinical characteristics, grade-IV), palpation of thyroid nodules and/or adjacent adenopathies (mobile/fixed, painful/not painful, reactive, etc.). In addition, it is necessary to pay special attention to the size and weight of the patient, heart rate and blood pressure, body temperature, skin adnexa such as hair and nails, skin charac-

In patients with thyroid hyperfunction, there is usually weight loss (accompanied by nausea, vomiting, diarrhea and often an increased appetite), excessive urination and thirst, along with

The cardiovascular system is altered by thyroid hormones which have important effects on cardiac muscle, the peripheral circulation, and the sympathetic nervous system. There is an important correlation between the hyperthyroid state and cardiac morbidity, with cause– effect determination. Cardiac symptoms such as tachycardia, heart failure, or arrhythmia and

About psycho-neurological manifestations, we have to highlight detected cases of tremors, chorea, myopathy, myasthenia gravis, ophthalmopathy (exophthalmos), delirium, emotional lability, psychosis, paranoia, irritability, exhaustion, depression and panic attacks among others.

Fine and brittle hair or a diffuse hair loss due to an acceleration of capillary cycles is common. The skin is usually smooth, thin, moist and hot, with marked redness of the palms of the hands and tendency to facial flushing, due in large part to heat intolerance. Loss of libido and

Some laboratory alterations in addition to the thyroid profile such as high blood sugar, low cholesterol or calcium-phosphorus metabolism's alterations (with osteoporosis tendency) can

Regarding pediatric age, a high index of suspicion is required due to its important effects on the organism. Thyroid hormones play an important role in the development of the central nervous system and growth. A situation of thyroid hyperfunction can interfere with growth

teristics or menstrual changes in cases of women of childbearing age.

amenorrhea are other alterations that can be generated over time.

**3. Clinical presentation**

remarkable associated hyperactivity.

atrial fibrillation are most frequent [3, 4].

be visualized.

The TSHR is a G-protein coupled receptor present in thyroid, lymphocytes, fibroblasts and adipocytes. The binding of TSH to TSHR results in signaling pathway downstream that results in actions of thyroid hormone production [2].

Approximately 94% of thyroid hormones are secreted by the thyroid gland as tetraiodothyronine (T4) and 6% as triiodothyronine (T3). T4 is catalytically converted to T3 (more metabolically active) in peripheral tissues by deiodinases enzymes. Both T4 and T3 are mostly bound to carrier thyroxine-binding globulin proteins (TBG) in the serum [3].

At the cellular level, the function of thyroid hormones is mediated by the free hormones (free T4 (fT4) and free T3 (fT3)), principally by the binding of triiodothyronine (T3) to its receptors. In addition, the subsequent expression of genes is regulated by the binding of the T3-receptor complexes to DNA. This is specifically important, for example, for those genes that regulate the calcium cycling in cardiac cells [3, 4].
