Hormones, Thyroid

This chapter deals largely with the action of thyroid hormones on bile acid metabolism, because few definitive studies with other hormones (e.g., pituitary, adrenal, gonadal) have been carried out so far. 

In most mammalian species, serum cholesterol concentrations vary inversely with thyroid function. However, the rate of cholesterol synthesis is reduced in the hypothyroid state and enhanced in hyperthyroidism. These apparently contradictory observations have been explained in terms of changes in steady-state conditions. For example, increased cholesterol synthesis in hyperthyroid animals would be expected to lead to hypercho- 

TAddress correspondence to Dr. Erwin H. Mosbach, Public Health Research Institute, 455 First Avenue, New York, N.Y. 10016. 

The effects of thyroid hormone on cholesterol metabolism have been studied in bile fistula rats Such preparations exhibit an inverse relationship between serum cholesterol levels and biliary cholesterol concentrations. The level of biliary cholesterol is directly related to thyroid activity, and the biliary excretion of cholesterol increases in the hyperthyroid state and decreases in the hypothyroid state (2,3). 

in the rat, cholesterol is eliminated largely in the form of bile acids, it was expected that bile acid secretion in bile would be increased in the hyperthyroid state. Early experiments to test this point indicated that biliary bile acid secretion was actually normal or below normal (2,3). These results can be explained in terms of the inadequate analytical procedures then in use. Only cholate secretion was measured, and the levels of cheno-deoxycholate were not taken into account. When both of these bile acids were determined, it was shown that, in the bile fistula rat, the total production of bile acids was about the same in the hyperthyroid as in the euthyroid state, and lower in the hypothyroid state (4). In addition, in the hyperthyroid state, the normal ratio of cholate/chenodeoxycholate was reversed from approximately 3 1 to 1 3—cholic acid synthesis was decreased, and chenodeoxycholic acid synthesis was increased two- to threefold (4). Identical results were obtained in the bile fistula rat treated with noncalorigenic doses of D-tri-iodothyronine (5,6), suggesting that these effects are not necessarily a function of the basal metabolic rate. 

Esterification of the carboxyl group is usually performed by heating with a 25% solution of HC1 in methanol at 70°C for 30 min and the residue is acylated with TFA anhydride [298,299], Although the acylation proceeds under very mild conditions (20°C) and satisfactory results have been obtained for micromole amounts with the use of the FID, decomposition of the derivatives during the process has been observed when working at the picomole level with the ECD. 

Hollander and co-workers [303—305] dealt with the problem in detail and developed a method for the isolation of hormones from blood, using Bio-Rad AG 50W-X2 (100—120 mesh) ion-exchange resin. Acylation with pivalic anhydride—methanol—triethylamine (20 1 1) was performed at 70°C for 10 min. The derivatives were purified with the aid of Amberlite IR-45 resin and benzene as a solvent. The dry residue was dissolved in 100 jul of benzene and 5 /il were injected directly on to a 60 cm X 4 mm I.D. column packed with 5% OV-1 on Chromosorb W HP after an isothermal period at 220°C for 12 min, the temperature was increased at 3°C/min up to 300°C. Calibration standards were injected immediately after the sample. Almost identical results were obtained for T3 by GC and radioimmunoassay [304], Other workers [306] applied the same procedure to the seeds and analysed pivalyl methyl esters of T3 and T4 on an 81 cm column packed with 3% of Dexsil on Chromosorb W HP at 305°C. 

HFB anhydride has been used for the acylation of methyl esters of thyroid hormones to a lesser extent, in spite of the fact that the resulting derivatives are sufficiently stable and the most volatile and provide the highest ECD response, to which the acyl moiety also contributes. Acylation is said to be completed within 5 min at 50°C [307] however, other workers [308] performed acylations at 60°C for 1 h with a mixture of HFB anhydride (400 /il) and acetonitrile (500 /il) per 1 mg of the substrate. The derivatives of all of the thyroid hormones were separated on a capillary column (20 m X 0.15 mm I.D.) coated 

Trimethylsilyl derivatives are prepared by treatment with BSA alone [310,311] or with the addition of TMCS [312,314] in a suitable solvent (acetonitrile, pyridine, tetra-hydrofuran) or even without a solvent. For completion of the reaction, 10—20 min at 50°C are necessary [312], but as little as 30 min at 150°C has been reported for a stoppered vial with the use of a solvent [311], BSA alone can be used to advantage if pico-mole amounts are to be derivatized. The reaction products are said to decompose in dilute solutions even though pure BSA is used for dilution. At concentrations around 1 ng//d, up to 40% decomposition of the products is observed if diluted with BSA— acetonitrile (1 4), 100% decomposition occurs in 20 min. Of other silylating agents, e.g., HMDS and TMCS have been tested, but conversion into derivatives was not complete [311]. Silicone stationary phases of the SE-30, OV-1 and similar types have been used in the analysis. In most instances, temperature programming is required. Using the FID (in almost all instances), the detection limit is about 20 ng for T4 and 5—20 ng for T3, whereas with the aid of an ECD amounts about two orders of magnitude smaller can be detected [310,314]. Fig. 5.21 demonstrates a typical separation of five iodoamino acids and Tyr on 0.5% of SE-30. 

The silylation procedure has been accepted as a routine method for the trace analysis of preparations of thyroid hormones and drugs containing them. Quantitative evaluation was achieved by using T2 as an internal standard [314], The method has not been applied to the analysis of hormones in serum. Silylation does not seem suitable for this purpose as the derivatives partially decompose if sub-nanogram amounts are injected. 

PEPTIDES, PROTEINS, AND OTHER AMINO ACID DERIVATIVES 

The two main hormones of the posterior pituitary are oxytocin, which contracts the smooth muscle of the uterus, and vasopressin,  

The interferons are a family of proteins secreted by animal cells in response to viral and parasitic infections, and are part of the host s defence mechanism. They display multiple activities, affecting the functioning of the immune system, cell proliferation, and cell differentiation, primarily by inducing the synthesis of other proteins. Accordingly, they have potential as antiviral, antiprotozoal, immunomodulatory, and cell growth regulatory agents. 

Primary drug Secondary drug Effect Mechanism Precautions 

THYROID HORMONES ANAESTHETICS -GENERAL-KETAMINE Cases of tachycardia and hypertension when ketamine was given to patients on thyroxine this required treatment with propanolol Uncertain Monitor PR and BP closely 

THYROID HORMONES ANTIARRHYTHMICS -AMIODARONE Risk of either under- or overtreatment of thyroid function Amiodarone contains iodine and has been reported to cause both hyper- and hypothyroidism Monitor triiodothyronine, thyroxine and TSH levels at least 6-monthly 

THYROID HORMONES QUINOLONES -CIPROFLOXACIN i levels of levothyroxine and possible therapeutic failure The mechanism has not been elucidated the concurrent administration of ciprofloxacin with levothyroxine may interfere with the absorption of levothyroxine and result in lower than expected levels The interaction may be minimized by separating dosing of the two agents ciprofloxacin should be taken several hours before or after taking levothyroxine 

THYROID HORMONES RIFAMPICIN 1 levothyroxine levels Induction of metabolism Monitor TFTs regularly and consider t dose of levothyroxine 

Levothyroxine Sodium (Synlliioid. Letter, Levoxine. Levoid) 

Levothyroxine sodium is used in replacement therapy of decreased thyroid function (hypothyn)idism). In general, a 

Liothyronine Sodium, USP. Lioihyroninc sodium. O-(4-hydroxy-3-iodophenyl)-3.5-dii )do-i.-lhyroxine monoso-dium. salt (Cylomel), is the sodium salt of L-3.3. S-triiodolhy-ronine. It occurs as a light-tan. odorless, crystalline powder, which is slightly soluble in water or alcohol and has a specific rotation of -H 18 to 22° in a mixture of diluted HCI and alcohol. 

Liothyronine sodium occurs in vivo together with levo-ihyroxinc sodium it has the same qualitative activities as thyroxine but is more active. It is absorbed readily from the gastrointestinal tract, is cleared rapidly from the bloodstream, and is hound more loosely to plasma proteins than is thyroxine, probably because of the less acidic phenolic hydroxyl group. 

SCS are the same as tho.se of levothyroxine sodium, including treatment of metabolic insufficiency, male infcnil-ity. and certain gynecological disorders. 

In order to avoid errors due to pathological conditions, the present review of the literature will be limited to observations made on nongoitrous subjects and animals for this reason, the average values reported in the review may in some cases deviate from those listed by the authors. 

Measurable amounts of iodine in the human thyroid gland have been detected at a fetal age of 17 weeks by Lelkes (1933) in Germany, Elmer and Scheps (1935) in Poland, and Widdowsen and Spray (1951) in England. In the German and Polish studies, higher iodine concentrations were recorded at a fetal age of 4-5 months (5 mg %) than at term (0.5 mg %), whereas the a.ssays made in England showed a progressive increase with embryonic age in the thjrroid iodine concentration. 

Mean Iodine Concentrations of the Thyroid Gland of Children to 15 Years of Age  

Authors Year Country Newborn 0.1-12 Months 1-4 Years 5-9 Years 10-15 Years 

Variation with age in iodine concentration of the thyroid gland in children and young adults. The figure is constructed on the basis of data reported by Widdow-sen and Spray (1951). 

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Brent et al., 1989] Brent, G. A., Dunn, M. K., Harney, J. W., Gulick, T., and Larsen, P. R. Thyroid hormone aporeceptor represses Ta inducible promoters and blocks activity of the retinoic acid receptor. New Biol. 1 (1989) 329-336 [Cevc and Marsh, 1987] Cevc, G., and Marsh, D. Phospholipid Bilayers Physical Principles and Models. John Wiley Sons, New York, 1987. 

Thyroid compounds Thyroid counter Thyroid gland Thyroid hormone... 

Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone. 

Thyroid-stimulating hormone can be used clinically to test thyroid function but has not found practical apphcation in the treatment of human thyroid insufficiency. Direct replacement therapy with thyroid hormone is easy and effective, owing to a simple molecular stmcture. TSH has been used in the veterinary treatment of hypothyroidism, and preparations of TSH ate produced by Cooper Animal Health, Inc. and Armour Pharmaceuticals. 

Metabolic Functions. The functions of the thyroid hormones and thus of iodine are control of energy transductions (121). These hormones increase oxygen consumption and basal metaboHc rate by accelerating reactions in nearly all cells of the body. A part of this effect is attributed to increase in activity of many enzymes. Additionally, protein synthesis is affected by the thyroid hormones (121,122). 

Thyroid Hormones. Iodine, absorbed as P, is oxidized in the thyroid and bound to a thyroglobulin. The resultant glycoprotein, mol wt 670,000, contains 120 tyrosine residues of which ca two-thirds are available for binding iodine in several ways. Proteolysis introduces the active hormones 3,5,3 -triiodothyronine (T ) and 3,5,3, 5 -tetraiodothyronine (T, (thyroxine) in the ratio Ty.T of 4 1 (121,122). 

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 class III cytokine receptor family includes two TNE receptors, the low affinity NGE receptor and 7-ceU surface recognition sites that appear to play a role in proliferation, apoptosis, and immunodeficiency. TNE-a (- 17, 000 protein) is produced by astrocytes and microglia and can induce fever, induce slow-wave sleep, reduce feeding, stimulate prostaglandin synthesis, stimulate corticotrophin-releasing factor and prolactin secretion, and reduce thyroid hormone secretion. TNE-a stimulates IL-1 release, is cytotoxic to oligodendrocytes, and reduces myelination this has been impHcated in multiple sclerosis and encephalomyelitis. Astrocyte TNE-a receptors mediate effects on IL-6 expression and augment astrocytic expression of MHC in response to other stimulants such as lEN-y. 

Thyroid hormone receptors (THRs) are subdivided intoa and P types, each having two isoforms. In rat brain, THR, mRNA is present in hippocampus, hypothalmus, cortex, cerebellum, and amygdala. Thyroxine (l-T (284) and triiodothyronine (l-T ) (285) are endogenous ligands for the THRs. TRIAC (286) is a THR antagonist. Selective ligands for PPARs have yet to be identified (Table 16). 

The main role of the human thyroid gland is production of thyroid hormones (iodinated amino acids), essential for adequate growth, development, and energy metaboHsm (1 6). Thyroid underfunction is an occurrence that can be treated successfully with thyroid preparations. In addition, the thyroid secretes calcitonin (also known as thyrocalcitonin), a polypeptide that lowers excessively high calcium blood levels. Thyroid hyperfunction, another important clinical entity, can be corrected by treatment with a variety of substances known as antithyroid dmgs. 

Fig. 1. Mechanisms controlling free thyroid-hormone levels.

Fig. 2. Early events in thyroid-hormone action. Interaction of T with cell nuclear receptors (6).

Myxedema and goiter are the main conditions for which thyroid preparations are indicated. The treatment of cretinism is difficult because it is recognized only at or after birth. Even if this disease could be diagnosed m utero, thyroid hormones do not readily cross the placental barrier. In addition, the fetus, as does a premature infant, rapidly deactivates the thyroid hormones. The halogen-free analogue DlMlT [26384-44-7] (3), which is resistant to fetal deiodinases, may prove useful for fetal hypothyroidism (cretinism). 

Structure—Activity Relationships. In spite of the considerable synthetic and bioassay effort involved in estabhshing the thyromimetic potency of thyroid-hormone analogues, more than 100 compounds have been studied (Table 2). The main stmctural requirements for thyromimetic activity can be summarized as follows (6,12—16). 

Chemical Assay. In view of the similarity of their chemical and physical properties (see Table 1) (29), the main problem in the chemical analysis of the thyroid hormones is their separation. A USP procedure gives the details of a paper chromatographic separation in which T is examined for contamination by T and 3,5-diiodothyroiiine (30). Other systems are also employed (29). 

The amphibian metamorphosis test is based on the abiUty of thyroid hormones to induce precocious transformation of a tadpole into a frog or of the axolod into a salamander. It is rarely used because of solubiUty problems and the difficulty of applying the results to humans. 

Using any of the carrier proteins available in highly purified form, eg, TBG or TBPA, a convenient and accurate quantitative determination of and is possible by displacement of radioiodinated or T. This procedure enables their quick determination at low concentrations even in the presence of coundess other substances that occur in body duids (31). In a similar fashion, intact cell nuclei or solubilized proteins from rat fiver cell nuclei, which display high affinities for thyroid hormones, especially T, have been used to establish relative binding affinities of many thyromimetic compounds (7). 

Iodide and Other Inorganic Anions. When large doses of iodide ion are administered, a transient inhibition of synthesis and release of the thyroid hormones is brought about by the so-called Wolff-Chaikoff effect. 

Thiocyanate ion, SCN , inhibits formation of thyroid hormones by inhibiting the iodination of tyrosine residues in thyroglobufin by thyroid peroxidase. This ion is also responsible for the goitrogenic effect of cassava (manioc, tapioca). Cyanide, CN , is liberated by hydrolysis from the cyanogenic glucoside finamarin it contains, which in turn is biodetoxified to SCN. 

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