DL-Thyroxine

DL -Thyroxine, needle -like crystals. Dec 231-233. Insol in water, in alcohol, and in the other usual organic solvents, but in the presence of mineral acids or alkalies it dissolves in alcohol sol in solns of the alkali hydroxides and in hot solns of the alkali carbonates. When alkali hydroxide solns of thyroxine are satd with sodium chloride, the sodium salt of thyroxine separates. 

Triiodothyronine and thyroxine are the physiologically active hormones of the thyroid gland. Tyrosine is the starting material for their synthesis. Gries and co-workers [62] have chromatographed thyroid-active iodoamino acids on cellulose G layers and obtained good separations of DL-thyroxine, DL-truodothyroxine, DL-diiodothyronine, dl-monoiodothyronine, diiodotyrosine and monoiodotyrosine. 

Hormone administration studies. An increase in hepatic G-6-Pase has been observed in animals made thyrotoxic by the injection of thyroxine over a period of several days (Glock et al., 1956 Harper, 1956 Maley and Lardy, 1955). Treatment of rats with 0.5 gm. DL-thyroxine per day for 8 days produced a 47 % increase in G-6-Pase activity per gram wet liver and 30 % of normal when expressed per total liver. 

Adverse effects on the thyroid have not been observed in children, however. In a study of inner-city children, linear regression analysis revealed that there was no association between PbB levels and either thyroxin or free thyroxin (Siegel et al. 1989). Similar findings were reported by Huseman et al. (1992) in a group of 12 children from the Omaha Lead and Poison Prevention Program with PbB levels in the range of 41 to 72 pg/dL. Siegel et al. (1989) offered four possible explanations to account for this apparent... 

Figure 19.1 Dose-dependent response curve for thyroxine (T4) assay using E5-Ab and double-antibody immune complex. Solutions (20 juL) of the reference samples containing 0.0 to 25.0 /ig/dL T4 were mixed with 180 fiL of the releasing solution. Fractions (132 juL) of this mixture were combined with 38 //Lof an E5-Ab solution. After standard incubation, 76 A. of this mixture was spotted on a blank Stratus tab. Incubation times and reagent volumes used for the rest of the reagents, such as the conjugate and the substrate wash, were identical to those defined for an existing T4 assay on Stratus ...

Heyma P, Larkins RG, Higginbotham L, Ng KW. D-pro-pranolol and DL-propranolol both decrease conversion of L-thyroxine to L-triiodothyronine. BMJ 1980 281(6232) ... 

Reference interval Every laboratory must determine its own reference interval, since the concentration of free thyroxine can differ regionally. 0.9-2.1 ng/dl have been stated as guidance values. 

Thyroxine-TSH. Normally, thyroxine sends a negative feedback to the pituitary and reduces the secretion of thyroid stimulating hormone (TSH). Patients with serum thyroxine concentrations above 8.0 ig/dL seldom have elevated concentrations of TSH. Exceptions occur in secondary and tertiary hyperthyroidism. 

Transthyretin amyloidosis (also called familial amyloid polyneuropathy) is an autosomal dominant syndrome characterized by peripheral neuropathy. This disease results from one of five mutations identified thus far in the gene for transthyretin. Transthyretin is also called prealbumin (although it has no structural relationship to albumin) because it migrates ahead of albumin in standard electrophoresis at pH 8.6. Transthyretin is synthesized in the liver and is a normal plasma protein with a concentration of 20-40 mg/dL. It transports thyroxine and retinol binding protein (Chapter 38). The concentration of transthyretin is significantly decreased in malnutrition and plasma levels are diagnostic of disorders of malnutrition (Chapter 17). 

Transthyretin (also known as prealbumin) Liver plasma circulating form is a tetramer composed of four identical monomers. M.W. 55,000 1-2 days N 20-40 mg/dl Mild 10-15 mg/dl Moderate 5-10 mg/dl Severe <5 mg/dl Circulates in plasma in a 1 1 complex with retinol-binding protein, transports thyroxine, has a small body pool, and has a short half-life. Sensitive indicator of protein deficiency and in the improvement with protein refeeding. 

Retinol-binding protein (RBP) Liver M.W. 21,000 10-12 hours N 3.5-9.0 mg/dl Circulates in plasma in 1 1 complex with transthyretin, transports retinol and thyroxine, plasma levels influenced by glomerular filtration rate, retinol and zinc status, considered to be too sensitive and therefore has limited value... 

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