**3. Clinical presentation**

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

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

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

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

Radiopharmaceuticals are substances that contain one or more radioisotopes (radionuclides). They are nonencapsulated sources of artificial ionizing radiation, which are used for both

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 radiophar-

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

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

diagnostic and therapeutic medical applications in the field of nuclear medicine.

results in actions of thyroid hormone production [2].

the calcium cycling in cardiac cells [3, 4].

**2. Radiopharmaceuticals principles**

maceuticals for thyroid imaging.

organification defects [5–7].

trapped and not organified in the follicles [7].

to carrier thyroxine-binding globulin proteins (TBG) in the serum [3].

isthmus [1].

32 Thyroid Disorders

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

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 remarkable associated hyperactivity.

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 atrial fibrillation are most frequent [3, 4].

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 amenorrhea are other alterations that can be generated over time.

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 be visualized.

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 and development, result in growth retardation, brain damage due to craniosynostosis and cognitive impairment [8].

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

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

http://dx.doi.org/10.5772/intechopen.77161

35

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

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

**5. Thyroid scintigraphy: patient preparation, instrumentation and image acquisition. Evaluation of thyrotoxicosis associated with** 

Usually no prior patient preparation is needed for thyroid scintigraphy [6]. It is not necessary to carry out any special diet or suspend the usual medication. In case the patient is taking thyroid hormone replacement therapy (levothyroxine), it is necessary to stop taking such

That medication can be restarted as usual once the image is acquired. If the patient is taking an iodine supplement or has recently had an intravenous iodine test (such as a CT scan with

On the other hand, women who are pregnant or breastfeeding should inform the nuclear medicine physician before any testing of the service. Although the exposure to the radiation involved is very low with 99mTc-pertechnetate, the benefit/risk of the test must be compared, using the lowest possible dose to obtain an adequate image (optimization cri-

Breastfeeding will be suspended for 24 h after performing the thyroid scan and the importance of drinking plenty of water for an early elimination of the radiotracer will be reported,

About radiation protection measures, it is recommended not to be in contact with pregnant women or young children for 24 h after the scintigraphy. If this condition cannot be met, a

detection of pharmacological response (drug resistance) or clinical relapse.

relationship with other autoimmune entities.

**thyroid hyperfunction (clinical examples)**

medication at least 30 days before the imaging study.

**5.1. Patient preparation, instrumentation and image acquisition**

intravenous contrast), the study should be delayed 4–6 weeks later [6].

which will reduce the exposure time of the embryo/fetus to radiation.

distance of at least 1 m from the patient should be maintained.

benign nodules [10].

terion) [6].

Although the manifestations are mostly similar to adults, the initial clinical presentation may be different in the pediatric age and even the symptoms may vary within this age group according to (prepubertal or pubertal population). A highlight of certain symptoms as an example of such atypical presentation and that are subject to confusion are mood changes and emotional lability, fatigue, sleep disturbance and increased appetite (prepubertal children more commonly present with poor weight gain and frequent bowel movements), attentiondeficit hyperactivity disorder, poor school performance, irritability, fatigue, palpitations, heat intolerance, fine tremor and a goiter [2].

Definitive diagnosis can be more challenging in pregnancy. A diffuse goiter, ophthalmopathy, hyperthyroid symptoms prior to pregnancy and serum thyroid hormone receptor antibody (TRAb) positivity favor the diagnosis of *Graves' disease*. Transient gestational thyrotoxicosis is more common in women with morning sickness, especially those with the most severe form, hyperemesis gravidarum [9].
