**3. Functions**

thyroid hormones and retinol-binding protein (RBP) [4]. Moreover, transthyretin is one of the precursors, which may be found in amyloid deposits [5]. In humans, analyses of the concentrations of TTR in serum are recommended by some investigators as a screening marker for inflammation, malnutrition, or both [6]. In animals, there are only scarce literature data about

Transthyretin is a small globular non-glycosylated tryptophan-rich protein of a homotetrameric structure, composed of four identical subunits with two thyroxine-binding sites per tetramer [8]. The binds for retinol-binding protein are placed at the surface of the molecule and do not interfere with thyroxin binding (**Figure 1**) [9]. Its molecular mass is of 54.98 kDa, which is small enough to penetrate the vascular wall and migrate into the extravascular space as easily as albumin or transferrin [10]. In some conditions, the transthyretin molecules may aggregate and form insoluble fibrillar deposits, which may be associated with amyloid diseases, predominantly senile systemic amyloidosis or neurodegenerative familial amyloidotic polyneuropathy [11, 12].

**Figure 1.** The three-dimensional structure of transthyretin displayed as a dimer (A) and a tetramer in complex with

thyroxine (B) and retinol-binding protein (C) [103].

this protein as a biomarker of health state and its use in the laboratory diagnosis [7].

**2. Structure**

52 Pathophysiology - Altered Physiological States

Transthyretin is a serum protein with multiple functional properties [13]. The main physio logical functions of TTR include the carriage of thyroid hormone and indirectly vitamin A, which may promote the maturation of lymphocytes [14, 15]. Although each monomer of the TTR molecule has two binding sites for thyroid hormones, the binding of one molecule of T3 or T4 may reduce the binding affinity for the second site [6]. Moreover, the binding affinity for T3 is lower compared with that for T4. Transthyretin binds and transports approximately 15–20% of thyroid hormones circulating in the serum and up to 80% of thyroxine in the central nervous system (CNS) [16]. About 70% of thyroid hormones are transported by thyroxin-binding globulin (TBG), which is the major serum transport protein in humans [17]. The remaining part of thyroid hormones is transported by albumin. These proteins are responsible for the transporting of thyroid hormones to cells and maintaining a large store of these hormones in the blood in a non-diffusible form [2]. Among animal species, the concentration of TBG in the dog is only 15% of those observed in humans [18]. Cats do not appear to have a high-affinity thyroid-binding protein such as TBG, but have only transthyretin and albumin [19]. Some other small molecules may bind in the thyroxine-binding sites of TTR, including some natural products, drugs or toxicants [20]. These interactions with TTR may be important when TTR becomes a major circulating thyroxine-binding protein, for example, in humans with complete or partial TBG deficiency, or when the concentration of thyroxine in the serum is markedly increased [21].

In addition to the binding and carriage of thyroid hormones, transthyretin has a more important function, that is, the transport of retinol (vitamin A) through its association with retinol-binding protein (RBP) from its main storage site in the liver to target cells [22]. Retinol is bound to RBP, and then RBP binds to transthyretin. This binding of RBP to TTR was suggested to prevent the extensive loss of RBP, which is of low molecular weight and would be rapidly eliminated from plasma by glomerular filtration if it were not complexed to transthyretin [23, 24]. Although each of the four monomers has a binding site for RBP, the tetramer binds only one molecule of RBP with high affinity, and possibly a second with lower affinity [25].

Moreover, transthyretin acts as a negative acute phase reactant, serum concentrations of which fall due to decreased synthesis in inflammation, trauma, tissue injury or stress [26].

#### **4. Synthesis**

Transthyretin is synthesized mainly by hepatic parenchymal cells and in the choroid plexus of the brain, which has the highest concentration of TTR in the body [27, 28]. In cerebrospinal fluid, it is the second most abundant protein, which may be involved in the pathogenesis of Alzheimer´s disease, depression and lead intoxication [29]. Other tissues have been reported also to produce TTR, but in much lower concentrations [30]. Small amounts of TTR are also produced by retinal pigment epithelium and the pineal gland [31]. Transthyretin has also been found in adult pancreatic islet cells, enterochromaffin cells in the gastrointestinal mucosa, as well as kidney cells [32, 33]. Neoplastic tissues, including choroid plexus papillomas, glucagonomas and gut carcinomas, have been reported also to secrete transthyretin [33]. During fetal life, TTR is synthesized by the embryonic yolk sac endothelium [34]. Any alteration in energy-to-protein balance impairs the body mass reserves and causes early depression in the production of transthyretin [35].

be expect that the newborn is in reasonable nitrogen balance and will gain weight subsequently. According to Benvenga et al. [49], the concentrations of TTR progressively decrease after 50–60 years. Approximately from the year 60 of age, muscle mass undergoes stepwise shrinking leading to sarcopenia, which may be responsible for the aforementioned decrease in the concentrations of TTR [50, 51]. In animals, the influence of age on the concentrations of TTR during the growth and development is not well described. A marked increase of TTR values from 72.9 to 251.4 mg/L was observed by Tóthová et al. [7] in calves 1 day after colostrum intake with a consecutive gradual decrease till the end of the third month of life. Rona [52] described that bovine colostrum contains among other bioactive molecules a small amount of prealbumin (transthyretin). Thus, the increase of serum TTR concentrations observed in calves after colostrum intake may reflect the adequate nutrition, as well as its hepatic synthesis due to adequate protein and energy intake [14]. In neonatal rats, low concentrations of TTR were found in the immediate postnatal period, which increases at the time when the concentrations of both thyroxine and corticosterone increase [53]. On the other hand, there were no significant differences in the serum concentrations of TTR in three different age groups of

Transthyretin in the Evaluation of Health and Disease in Human and Veterinary Medicine

http://dx.doi.org/10.5772/intechopen.68725

55

The concentrations of transthyretin linearly increase after birth without marked sexual differences during infant growth [55]. During human puberty, major hormonal and metabolic alterations occur, which result in increased height, weight gain and a substantial redistribution of body tissues. While androgens promote the development of muscle mass in male teenagers, oestrogens contribute to minimal enlargement of the female musculature and stimulate the accretion of subcutaneous fat depots [56]. These differences lead to higher concentrations of TTR in male adolescents compared with values recorded in teenage girls [57]. Higher concentrations of TTR in males compared with females were found also by Benvenga et al. [49] and Gaggiotti et al. [58]. Studies dealing with the evaluation of gender-

Pregnancy, hormonal changes, physiological status and stress are other factors that may influence the concentrations of transthyretin. In humans, the concentrations of TTR were evaluated by Zhu et al. [59] during normal pregnancy. The values of TTR increased significantly in the third month of gestation and rapidly decreased following 20 weeks of gestation. Transthyretin was measured also in females with severe preeclampsia, showing significantly decreased TTR concentrations in these patients compared with the control group. Transthyretin is synthesized also by placental trophoblasts, which are critical to the normal fetal development. Thus, disorders caused by the production of TTR may result in fetal distress [60]. The aforementioned results indicated that TTR may be a reliable biomarker for the diagnosis of severe preeclampsia [59]. Similarly, Kalkunte et al. [61] suggested a relationship between reduced TTR production and preeclampsia. The importance, functional role and alterations in the concentrations of TTR during pregnancy in animals have not been reported. Our findings suggest no significant changes in TTR concentrations during the last week of pregnancy and early stages of lactation in dairy cows (unpublished data). However, further evaluations are needed to establish the values of transthyretin and its possible changes in pregnant and lactating cows, which may be useful for veterinary practitioners in the early diagnosis, prevention and finding

related differences in the concentrations of TTR in animals were not found.

therapeutic solutions in periparturient dairy cows.

pigs from 10 to 25 weeks [54].

The major sites of transthyretin degradation are the liver, muscles and skin, but a small amount of TTR may be catabolized by other tissues, including kidneys, adipose tissues, testes, as well as the gastrointestinal tract [36]. Transthyretin has a half-life in plasma of approximately 2 days, which is much shorter than that of albumin [37]. Transthyretin is therefore more sensitive to changes in protein-energy status, but its concentrations closely reflect the recent nutritional status rather than the overall nutritional support [38, 39].

#### **5. Laboratory assays**

Transthyretin is considered a more sensitive indicator of visceral protein status than albumin and transferrin because of its short half-life and low concentration in the body [40]. Conventionally, radial immunodiffusion and electroimmunodiffusion have been used for routine determination of transthyretin in humans [41]. Faster and more precise immunonephelometric and immunoturbidimetric assays have been developed also, which are easily applicable to many laboratory-automated equipments available in hospitals [6, 42]. Moreover, a sensitive enzyme immunoassay (enzyme-linked immunosorbent assay (ELISA)) for the determination of TTR values has been described, but this method is more time consuming and expensive compared with the above mentioned, and is more applicable, for example, for the estimation of TTR in the cerebrospinal fluid in nanogram amounts [43]. In veterinary medicine, species-specific ELISA is the most common analytical tool for the detection and quantification of transthyretin, utilizing monoclonal anti-TTR antibodies. In some avian species, for example, in budgerigars (*Melopsittacus undulatus*) transthyretin (prealbumin) constitutes as high as 75% of the total albumin concentration [44]. Therefore, it may be visualized and quantified easily by protein electrophoresis.
