Thyroxine, small amounts

Iodine occurs to a minute extent (less than 0.001 %) in sea water, but is found in greater concentration, combined in organic form, in certain seaweeds, in oysters and in cod livers. Crude Chile saltpetre, or caliche contains small amounts of sodium iodate, NalOj. from which iodine can be obtained (see below). Some insoluble iodides, for example liiose of silver and mercury(II), occur in Mexico. Iodine is found in the human body in the compound thyroxin in the thyroid gland deficiency of iodine in diet causes enlargement of this gland (goitre). 

Only small amounts of free T are present in plasma. Most T is bound to the specific carrier, ie, thyroxine-binding protein. T, which is very loosely bound to protein, passes rapidly from blood to cells, and accounts for 30—40% of total thyroid hormone activity (121). Most of the T may be produced by conversion of T at the site of action of the hormone by the selenoenzyme deiodinase (114). That is, T may be a prehormone requiring conversion to T to exert its metaboHc effect (123). 

The substance to be assayed—e.g., the hormone thyroxine in a serum sample—is pipetted into a microtiter plate (1), the walls of which are coated with antibodies that specifically bind the hormone. At the same time, a small amount of thyroxine is added to the incubation to which an enzyme known as the "tracer" (1) has been chemically coupled. The tracer and the hormone being assayed compete for the small number of antibody binding sites available. After binding has taken place (2), all of the unbound molecules are rinsed out. The addition of a substrate solution for the enzyme (a chromogenic solution) then triggers an indicator reaction (3), the products of which can be assessed using photometry (4). 

Iodine is concentrated in humans by the thyroid gland to form the iodo-amino acid thyroxine, which is essential to normal health and development. Iodine is a rather rare element (crustal abundance 0.00003 weight %, cf. Table 1.1), so the thyroid gland has become very efficient at scavenging iodide ion. As iodine is deficient in the diet in some locations, a small amount of iodide ion is routinely added to commercial table salt ( iodized salt ). 

In these discrete glands, a series of specihe chemical reactions can be demonstrated. Sec Fig. I. Some of these reactions take place in the absence of any apparently specihe synthesis of iodotyrosines. and it has not been fully explained how many of these steps require enzymes, even in mammalian forms, although these processes arc usually considered us enzymatic. It is possible in iodinaic tyrosine in soluble proteins in vitro by addition of elemental iodine and under these circumstances thyroxine and uiiudoihyroxine will also be formed, in company with small amounts of lodnhistidine. 

The effects of iopromide on thyroid function have been investigated in 20 pre-term infants with very low birth weights and 26 matched premature infants who did not receive contrast medium (571). The dose of iopromide (iodine 300 mg/ml) was 0.3-1.0 ml. Iopromide did not affect the concentrations of free thyroxine and thyroid stimulating hormone. This was attributed to the small amount of free iodide that iopromide contains... 

The thyroid gland secretes two hormones, thyroxine (3,5,3, 5 -L tetraiodothyronine) and triiodothyronine (3,5>3 -L-triiodothyronine), which are commonly known as T4 and T3, respectively (Table 52-1). In addition, the thyroid gland secretes small amounts of biologically inactive 3,3, 5 -L-triiodothyronine (reverse T3 [rTa]) and minute quantities of monoiodotyrosine (MIT) and diiodotyrosine (DIT), which are precursors of T3 and T4. The structures of these compounds are shown in Figure 52-1. 

In 1939, von Mutzenbecher showed that, if diiodotyrosine was incubated at an alkaline pH under aerobic conditions, small amounts of thyroxine were formed. This work gave the first direct experimental evidence that the reaction postulated by Harington could take place. Later, with the aid of radioactive iodine it was shown by the studies of Chaikoff and of Leblond and their collaborators that this reaction does indeed take place and it is now generally accepted that diiodotyrosine is the precursor of thyroxine in the thyroid. The large amount of experimental work which has contributed to this knowledge has recently been reviewed (Roche and Michel, 1951 Albert, 1952 Gross and Pitt-Rivers, 1952c) and will not now be considered in detail. Instead we will consider the means whereby the reaction is achieved in the body. 

Gross, Leblond, Franklin, and Quastel (1950) have shown that rat thyroids contain small amounts of free iodinated amino acids which can be extracted with butanol from the thyroid without previous hydrolysis. These include monoiodotyrosine, diiodotyrosine, thyroxine, and triiodothyronine, and they are presumably formed by proteolysis of thyroglobulin. Their fate has been the subject of much experimental work in recent years. 

A common use of iodine is as KI in table salt. Iodized salt provides the small amount of iodine necessary in our diets it is essential for the formation of thyroxin, a hormone secreted by the thyroid gland. Lack of iodine in the diet results in an enlarged thyroid gland, a condition called goiter. 

Mono- and dithyronine never reach the general circulation in appreciable amounts because these compounds are rapidly deiodinated thyroxine and 3,3, 5-triiodothyronine are found in blood not in a free form, but complexed to proteins. Among the two iodinated thyronines, thyroxine is the principal circulating hormone, and triiodothyronine is present only in small amounts. 

The blood, the interstitial fluid, the lymph, and the cerebrospinal fluid contain small amounts of free thyroxine. The role of the free hormone is not clear. The fact that it increases in thyrotoxicosis indicates that it may play a role in physiology. 

Changes in oxidative phosphorylation seem to be more closely related to the primary site of action of the thyroid hormones. Thyroxine uncouples oxidative phosphorylation in mitochondria and causes the energy released during oxidation of the Krebs cycle intermediates to be less efficiently used for ATP synthesis. A direct effect of thyroxine on oxidative phosphorylation would conveniently explain many metabolic effects of thyroxine, but it would leave unexplained the beneficial effects of small amounts of thyroxine. A direct effect on oxidative phosphorylation is also inconsistent with the fact that uncoupling agents are unable to correct hypothyroidism. 

Dialysis of blood through cellophane followed by chromatographic analysis of the dialysate has permitted the demonstration of small amounts of thyroxine in blood (approximately 6.5 pg of iodine per 100 ml). In thyrotoxicosis, the concentration of free thyroxine in blood increases to as high as five times normal. When thyroxine is administered intravenously to... 

Certain species of tadpoles excrete ammonia during the premetamorphic phase and urea after the onset of metamorphosis. In those animals, two of the urea cycle enzymes are present in small amounts during the premetamorphic phase—carbamyl phosphate synthetase and arginine synthetase. Metamorphosis can be induced by the administration of thyroxine, and... 

Adamson et alM recommended a colorimetric method based upon nitrosation for the determination of small amounts of thyroxine. Extraction of thyroxine from tablets for this method presents little difficulty most of the commonly used excipients are either soluble in the reagents without interfering or can be removed by some simple extraction and filtration technique. 

Mice are pretreated with to label the thyroid, 1 yg thyroxine, and codeine prior to injection of the material to be tested. The test material must be free of TSH, vasopressin, and MSH, which exert effects directly on the thyroid (R4). Codeine is given to sensitize (R4) the pituitary, and the small dose of thyroxine is given to partially suppress endogenous TSH secretion and establish basal conditions. Blood levels obtained 2 hours after injection are proportional to the amount of TRF injected (R4). Plasma TSH can be measured directly by use of the McKenzie assay (Mil), but this requires another group of animals. Pituitary TSH depletion can be measured directly by biossay of TSH in the pituitary of treated mice (S3). TSH assays will be discussed in Section 3.4. 

Before dismissing the possibility that triiodothyronine is formed by enzymic dehalogenation in the thyroid as well as in other tissues, it must be remembered that the amount of triiodothyronine present in the thyroid is very small compared with the amount of thyroxine. It is therefore possible that more sensitive methods for the detection of... 

Microinjections of amounts of thyroxine too small to have a systemic effect on the adenohypophysis or the anterior hypothalamus reduce TSH secretions. The effect is immediate when thyroxine is injected in the hypophysis. It is delayed when thyroxine is injected in the hypothalamus, but the minimal dose necessary to inhibit TSH secretion is the same whether thyroxine is injected in the hypothalamus or the hypophysis. 

Weigh an amount of powdered tablets equivalent to about 1-5 mg of thyroxine sodium into a nickel crucible about 6 mm in diameter and about 6 mm deep and add 3 ml of water followed by 5 g of sodium hydroxide in small pieces. Heat on a water-bath to remove the water, add a further 4 g of sodium hydroxide and heat over a Bunsen flame, at first gently and then sufficiently strongly to maintain the mixture in a... 

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