Thyroid gland activity

Weanling males fed diets containing 1500 mg KCN/kg, or 2240 potassium thiocyanate (KSCN) for 50 weeks No deaths or clinical signs of toxicity. Both groups had decreased thyroid gland activity. Cyanide, but not thiocyanate, caused reduction in growth rate 38... 

Diiodotyrosine does not possess pronounced hormonal activity. However, it stops production of thyrotropic hormone by the anterior lobe of the hypophysis, which activates thyroid gland activity. 

Radioactively labelled technetium ( " Tc) can be used instead of radioactive iodine (see radioactive iodine neck uptake test) for the assessment of thyroid gland activity. Its half life is six hours and thus the radiation dose to the patient is less than that given... 

The term chiral recognition refers to a process m which some chiral receptor or reagent interacts selectively with one of the enantiomers of a chiral molecule Very high levels of chiral recognition are common m biological processes (—) Nicotine for exam pie IS much more toxic than (+) nicotine and (+) adrenaline is more active than (—) adrenaline m constricting blood vessels (—) Thyroxine an ammo acid of the thyroid gland that speeds up metabolism is one of the most widely used of all prescription... 

Certain inorganic monovalent anions, similar in size to I, are also taken up by the thyroid gland and competitively inhibit active iodide transport with the following decreasing potencies ... 

Sodium Levothyroxine. As one of the active principles of the thyroid gland, sodium levothyroxine [55-03-8] (levothyroxine sodium) can be obtained either from the thyroid glands of domesticated animals (10) or synthetically. It should contain 61.6—65.5% iodine, corresponding to 100 3% of the pure salt calculated on an anhydrous basis. Its chiral purity must also be ascertained because partial racemi2ation may occur during synthesis and because dl-T is available commercially. Sodium levothyroxine melts with decomposition at ca 235°C. It is prepared as pentahydrate [6106-07-6] from... 

Three hormones regulate turnover of calcium in the body (22). 1,25-Dihydroxycholecalciferol is a steroid derivative made by the combined action of the skin, Hver, and kidneys, or furnished by dietary factors with vitamin D activity. The apparent action of this compound is to promote the transcription of genes for proteins that faciUtate transport of calcium and phosphate ions through the plasma membrane. Parathormone (PTH) is a polypeptide hormone secreted by the parathyroid gland, in response to a fall in extracellular Ca(Il). It acts on bones and kidneys in concert with 1,25-dihydroxycholecalciferol to stimulate resorption of bone and reabsorption of calcium from the glomerular filtrate. Calcitonin, the third hormone, is a polypeptide secreted by the thyroid gland in response to a rise in blood Ca(Il) concentration. Its production leads to an increase in bone deposition, increased loss of calcium and phosphate in the urine, and inhibition of the synthesis of 1,25-dihydroxycholecalciferol. 

Both drugs, MMI and PTU, are actively concentrated by the thyroid gland. Intrathyroidal concentrations of MMI are in the range of 5 x 105 M. There is no difference in intrathyroidal concentrations of MMI 3-6 and 17-20 h after ingestion of 10 mg of carbimazole. Little is known about intrathyroidal concentrations of PTU. Eight hours after a single dose of 10 mg of MMI or 100 mg of PTU inhibition of intrathyroidal organification of iodide is about 90% and 60%,... 

Decreased activity of the thyroid gland results in hypothyroidism and, in severe cases, myxoedema. It is often of immunological origin and the manifestations are low metabolic rate, slow speech, lethargy, bradycardia, increased sensitivity to cold, and mental impairment. Myxoedema includes a characteristic thickening of the skin. Therapy of thyroid tumours is another cause of hypothyroidism. Thyroid deficiency... 

Half-lives span a very wide range (Table 17.5). Consider strontium-90, for which the half-life is 28 a. This nuclide is present in nuclear fallout, the fine dust that settles from clouds of airborne particles after the explosion of a nuclear bomb, and may also be present in the accidental release of radioactive materials into the air. Because it is chemically very similar to calcium, strontium may accompany that element through the environment and become incorporated into bones once there, it continues to emit radiation for many years. About 10 half-lives (for strontium-90, 280 a) must pass before the activity of a sample has fallen to 1/1000 of its initial value. Iodine-131, which was released in the accidental fire at the Chernobyl nuclear power plant, has a half-life of only 8.05 d, but it accumulates in the thyroid gland. Several cases of thyroid cancer have been linked to iodine-131 exposure from the accident. Plutonium-239 has a half-life of 24 ka (24000 years). Consequently, very long term storage facilities are required for plutonium waste, and land contaminated with plutonium cannot be inhabited again for thousands of years without expensive remediation efforts. 

On the other hand, some receptors are truly promiscuous in that they can activate two or more G-proteins from quite different classes, even in their normal cellular environment. For example, similar concentrations of thyroid-stimulating hormone (TSH 0.1-100 U/ml) can stimulate the incorporation of 32P-GTP into a, aQ, a12, a13, as, and aq/11 through activation of the thyrotropin receptor in membranes from human thyroid gland. TRH activation of Ca2+ currents in GH3 cells is obtunded equally by antisense-depletion of l2, aa, and aq/11, but not of aQ. Some individual genotypic P2y nucleotide receptors can also couple with equal affinity to PTx-sensitive and PTx-insensitive G-proteins in sympathetic neurons. The degree, or otherwise, of such promiscuity is presumably determined by the structure of the receptor protein itself. 

Derived from the amino acid tyrosine, thyroid hormones are unique because they contain iodine. At this time, its incorporation into thyroid hormones is the only known use for iodine in the body. There are two thyroid hormones, named for the number of iodides added to the tyrosine residues of the thyroglobulin triiodothyronine (T3) and tetraiodothyronine (T4, thyroxine). Although significantly more T4 is synthesized by the thyroid gland, T3 is the active hormone. At the target tissue, T4 is deiodoninated to form the more potent T3. 

Calcitonin. This hormone, which is also secreted from the thyroid gland, is synthesized by the parafollicular cells (C cells) located between the follicles. The primary effect of calcitonin is to decrease the blood levels of calcium and phosphate. The mechanism of action involves the direct inhibition of osteoclast activity, which decreases bone resorption. This results in less demineralization of the bone and therefore a decrease in the release of calcium and phosphate from the bone into the blood. Calcitonin has no direct effect on bone formation by osteoblasts. 

The concentration of Li+ in the thyroid is three to four times that in serum [179]. It is thought that Li+ may be concentrated in the thyroid gland by a mechanism similar to the incorporation of iodide, I-, resulting in competition between Li+ and I the levels of intracellular 1 decrease when those of Li+ increase, and vice versa [182]. Li+ inhibits both the ability of the gland to accumulate 1 and the release of iodine from the gland. In vitro, Li+ has no effect on thyroid peroxidase, the enzyme that catalyzes the incorporation of I" into tyrosyl residues leading to thyroidal hormone synthesis, but does increase the activity of iodotyrosine-deio-dinase, which catalyzes the reductive deiodination of iodotyrosyls, thus maintaining the levels of intracellular I [182]. The increase in iodoty-rosine-deiodinase activity is probably a response to the Li+-induced decrease in the concentration of thyroidal I". Li+ has no effect on the conversion of thyroxine to triiodothyronine. The overall effect of this competition between Li+ and 1 is, therefore, reduced levels of thyroid hormone in the presence of Li+. 

T4 is secreted solely from the thyroid gland, but less than 20% of T3 is produced there the majority of T3 is formed from the breakdown of T4 catalyzed by the enzyme 5 -monodeiodinase found in peripheral tissues. T3 is about five times more active than T4. 

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