Serum Transthyretin

Serum Transthyretin
Transthyretin, known also as prealbumin or thyroxine-binding prealbumin, serves as a transport protein for thyroxine, and as a carier protein for RBP (Section 16.2.4). It has a longer half-life (i.e., 2d) (table 16.3) and a slightly larger body pool (0.010 g/kg body weight), compared to RBP, although the sensitivities of these two serum proteins to protein deprivation and treatment are similar. Transthyretin has a high content of the amino acid tryptophan and a high proportion of essential to nonessential amino acids. Thus it can be used as an indicator of the availability of essential amino acids in the body (Spiekerman, 1993.).
Serum Transthyretin can be used in hospital settings as a screening tool to identify patiens at risk for protein-energy malnutrition; more details are given in Section 27.1. Potter and Luxton (1999) examined the results of using Transthyretin as a routine diagnostic test for protein-energy malnutrition in emergency room admissions. They emphasized that Transthyretin was a more sensitive indicator of protein-energy malnurition than was serum albumin and was significantly associated with length of hospital stay.

Serum Transthyretin responds rapidly to the short-trem effects of nutritional therapy. As a result, serum Transthyretin is also used to monitor the progress of patiens receiving total parenteral nutrition postoperarively and during the transition from total parenteral nutrition to oral or enteral feeding (Winkler et al., 1989). Table 16.10 shows results for plasma RBP, Transthyretin, transferrin, and albumin in 68 patiens receiving total parenteral nutrition (TPN). Improvements in serum Transthyretin and RBP occurred within the first week of TPN and persisted throughout its duration, whereas there was no improvement in transferrin until the end of the therapy. In contrast, there were no significant differences in mean serum albumin levels between the pre- and post-TPN levels (Winkler et al., 1989).










Table 16.10. Plasma protein concentrations during total parenteral nutrition (TPN). Values are percentages of patiens (n = 68) who have normal concentrations of the proteins. *p < 0.05 vs. pre-TPN by chi-square. From Winkler et al., Journal of the American DieteticAssociation 89: 684-687, 1989 @ with permission of the Anerican Dietetic Association.

Other investigator have also reported a rapid rise in serum Transthyretin levels in contrast to the much longer time required to show an increase in serum albumin. Indeed, Bernstein et al. (1995) reported weekly increases in serum Transthyretin of 40-50 mg/L in response to adequate nutritional support and suggested that a response of less than 20 mg/L in a week is indicative of either inadequate nutritional support or a failure to respond to the treatment.
Deficiencies of vitamin A, zinc and iron do not affect the levels of Transthyretin, unlike those of RBP and tranferrin, as noted previously. In contrast, the presence of other conditions-such as gastrointestinal diseases, hepatic and kidney diseases, surgical trauma, stress, inflammation, and infection-leads to modifications in the metabolism of Transthyretin and reduces its specificity as an index of protein status. Nevertheless, hepatic disease does not affect Transthyretin as early or to the same extent as the other serum proteins, particulary RBP. Further, although serum Transthyretin is moderately elevated in renal diseases due to decreased catabolism by the kidney, nonetheless, in patients with stable renal failure, the direction of change in Transthyretin concentration rather than the equacy of nutritional therapy (Winkler et al., 1989).
The response of serum transthyretin concentrations to dietary protein intake has also been investigated. Shetty et al. (1979), reported that serum transthyretin concentrations did not respond to a short-term (10-d) protein restriction if energy intake was maintained, but levels fell markedly when both energy and, to a lesser extent. Protein intakes were restricted (Table 16.9). Ramsey et al. (1992) suggested that because of its very short half-life, serum transthyretin probably reflects acute protein intake rather than risk of protein malnutrition, which occurs over about 7 to 10 d. Nevertheless, Ogunshina and Hussain (1980) noted an inverse relationship of plasma transthyretin concentrations to the severty of malnutrition as assessed by the anthropometric classification of Waterlow (1972) (Figure 16.3 Section 13.2.5).












Figure 16.3 Plasma transthyretin (mean  SE) levels in children classified as normal or with mild, moderate, and severe protein-energy malnutrition according to the Waterlow classification of protein-energy malnutrition. From Ogunshina and Hussain, American Journal of Clinical Nutrition 33 : 794-800, 1980 @ Am J Clin Nutr. American Society for Clinical Nutrition.


















Table 16.11 : Reference medians and selected percentiles for serum transthyretin (g/L). stratified by age and gender. Data from Ritchie et al., Journal of Clinical Laboratory Analiysis 143 : 273-279, 1999 @ Wileyliss Inc., with permission.

Certain medications can influence serum transthyretin levels. Levels rise with anti inflammatory medications including some over-the-counter products Ingelbleek and Young, 1994). They are also altered by endogenous estrogen (e.g., during pregnancy) (Haram et al., 1983) or the use of estrogen containing preparations (e.g., oral contraceptive agents, estrogen replacement therapy).
Transthyretin values vary according to age and sex (Ritchie et al., 1999b). Values rise until the second or third decade of life and then decrease thereafter (Table 16.11). Males have higher values than females during midlife, this difference being highest at about age 40 y (i.e., 16%).

Interpretive Criteria
Interpretive values for adults based on neph elometry and the extent of the protein deficit are given in Table 16.8. Nutrition risk is high when serum transthyretin concentrations fall below 0.11 g/L, whereas poor outcome is predicted when a level < 0.50 g/L is observed (Logan and Hildebrandt, 2003).
Table 16.11 presents selected percentiles for serum transthyretin by age and gender, derived from the same Caucasian cohort used for both serum albumin and serum transferring (Ritchei et al., 1999b).

Measurement of Serum Transthyretin
Serum levels of transthyretin are four to five times higher than those of RBP and thus are easier to measure. Radial immunodiffusion techniques can also be used to determine transthyretin using commercial assay kits, as described for transferring and RBP. Other more sensitive and nephelmetry and turbimetry, both of which can operate on large automated chemistry analyzers.

Insulin-Like Growth Factor I
Insulin-Like growth factors are growth-hor-mone-dependent serum growth factors which are produced mainly in the liver. They have a pro-insulin-like structur and board anabolic properties. The insulin-like growth factors I (IGF-I), sometimes referred to as somatomedin-C, circulates bound to carrier proteins and has a half-life of 12-15 h (Thisen et al., 1994).
In studies of children suffering chronic undernutrition, decreased concentrations of circulating IGF-I in the serum occur, which respond rapidly to dietary treatment (Smith et al., 1981). When acutely malnourished patiens received nutritional support for 3-16 d, IGF-I levels increased from initial levels, although no significant changes in serum albumin, transferring, RBP, or transthyretin concentrations occurred (Unterman et al., 1985). These results suggest that serum IGF-I may be more sensitive to acute changes in protein status than are the other serum proteins. Further, serum IGF-I levels are not subject to diurnal variation and are not ingluenced by stress, sleep, or exercise, although levels are decreased in patients with hypothyroidism and with estrogen administration. in patients with liver disease, kidney failure, and several autommune diseases, serum IGF-I levels are very variable, unless the carrier protein is completely removed by the acid chromatography exraction method, as noted below.
More studies are necessary to establish the sensitivity and specificity of IGF-I measurements as an index of malnutrition before itcan be used as a routine biochemical marker of protein status and as marker for the response to nutritional therapy.

Measurement of insulin-like growth factor I
IGF-I can be assayed using commercially available radioimmunoassay kits, provided a gamma counter is available. Quality controls are provided with the assay kits. Other immunoassays inculuding immunoradiometric assays, enzyme-linked immunsorbent assays (ELISAs), and immunofunctional assays are also used. A number of different commercial kits are now available (Popii and Bauman, 2004).
Prior to the assay of IGF-I, any interfering binding proteins must be removed from the sample by an extraction procedure. Either acid-gel chromatography or acid-ethanol extraction is the most frequently used method. Of these, acid-gel chromatography is more difficult technically, but it is preferred because it removes 98% of the binding protein with a recovery of IGF-I of approximately 75%.
Either serum or plasma specimens can be used for the assay,but they separated as soon as possible after collection. Serum must be frozen promptly after separation to avoid falsey high values (Isley et al., 1990). Samples can be frozen at - 200C, but repeated freezing and thawing must be avoided.

Plasma Alkaline ribonuclease activity and fibronectin.
The activity of plasma alkaline ribonuclease (EC 3.1.4.2.2) has been investigated as a measure of protein status. Enzyme activity is reportedly increased in infants and children with protein malnutrition but returns to normal within 2-4 wk after rehabilitation (Scott et al., 1984). The measurement may be especially useful for monitoring the response of malnourished patients to nutrition interventions. A method for measuring alkaline ribonuclease activity in serum has been developed by Scoot (1979).
Young et al. (1990) suggest that concentrations of fibronection may also have potential as an index of protein status, and this warrants further study. Fibronectin is a glycoprotein which, unlike other serum proteins discussed above, is not synthesized exclusively by the liver but also by the endothelial cells, peritoneal macrophages, and fibroblasts. It has a half-life of about 15h. Fibronectin serves numerous physiological functions, including cell-matrix interactions, and the binding of macrophages and fibroblasts. In protein malnutrition, levels of fibronectin are reduced, but they return to normal rapidly after nutritional rehabilitation (Buonpane et al., 1989). Fibronectin, like the other serum proteins shown in Table 16.3, alo responds to certain non-nutritional factors, including infection, trauma, and burns, but again returns to normal on recovery. Special precautions must be taken when collecting serum samples for the assay of fibronectin. Analysis can be performed by competitive enzyme immunoassay (Ylatupa et al., 1993).

Metabolic changes as indices of protein status
Several striking changes in metabolism can develop in response to a reduced or inadequate supply of dietary protein or of some specific indispensable amino acids, the changes have been reviewed by Young and Marchini (1990). Of these, changes in free amino acid profiles in plasma have been described in children with frank kwashiorkor that are much more pronounced than those for children with the marasmic form of protein energy malnutrition. Such changes have been linked to the hyperisulinemia that occurs in kwashiorkor, which, in turn, results in characteristic alterations in serum amino acid concentrations via its effect on muscle protein sythesis (Coward and Lunn, 1981). Other metabolic changes, such as reduced urinary hydroxyproline excretion and increades urinary nitrogen excretion, occur in both the kwashiorkor and the marasmic forms of protein energy malnutrition, and these changes are used as less-specific indices of protein energy malnutrition.

Plasma amino acid ratio
Concentrations of free amino acids in plasma have been extensively studied in children with maramus and frank kwashiorkor. Earlier work suggested that the serum amino acid profile of children with frank kwashiorkor was markedly abnormal (Coward and Lunn, 1981). Concentrations of nonessential amino acids (NEAA) such as alanine, glycine, serine, and praline were elevated, whereas those for the essential (indispensable) amino acids (EAA) were lowered, resulting in a high NEAA to EAA ratio. Such changes in serum amino acid profiles were much less pronounced in marasmic children.
Based on apparent differences in serum amino acid profiles, Whitehead and Dean (1964) developed a simplified technique for use in the field to determine serum amino acid ratios using one dimensional paper chromatography and a fingerprick blood sample. The method was devised in an effort to distinguish between subclinical kwashiorkor and marasmus. Unfortunately, plasma NEAA, EA ratios didn’t show a consistent response in relation to the type or severity of protein energy malnutrition. Saunders et al. (1967) reported that the amino acid profile reflected the dietary protein intake immediately prior to the test and plasma amino acid profiles are no linger used to assess proein statu.