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thyroid pruoit
final
| Question | Answer |
|---|---|
| Hypothyroidism: Is more common endocrinopathy in the dog, & is diagnosed with some frequency in the | horse, but rare in cats & other domestic animals. |
| Less common but can be caused by iodine deficiency or ingestion goitrogenic substances in the environment or food | Hypothyroidism. |
| Goitrogenic substances are compounds that interfere with | thyroid hormone synthesis by the thyroid gland. |
| Surgical removal of | thyroid gland. |
| Hypothyroidism in dogs due to a defect within the | thyroid gland itself (primary hypothyroidism) is more common. |
| Two most common causes are | Lymphocytic thyroiditis & Idiopathic atrophy of the thyroid gland. |
| Rarely, hypothyroidism due to deficiency of | THS secretion have been reported. |
| Feline hypothyroidism is most common | after removal of thyroid gl& or after radio-iodine treatment for hyperthyroidism. |
| Hyperthyroidism is most common in | cats, & only occasionally seen in other domestic species. |
| Hyperthyroidism In cats usually due to | thyroid adenoma (adenomatous hyperplasia) & in dogs adenocarcinomas. |
| Some of the clinical signs of excess thyroid hormones are | Weight loss, ravenous appetite, hyperactivity, polydipsia, polyuria, diarrhea, intermittent fever, vomiting, cardiovascular effects (tachycardia) & dyspnea, etc. |
| Rarely- anorectic condition may be seen in | Hyperthyroidism. |
| Thyroid Hormone Synthesis,Thyroglobulin (TG) | a large glycoprprotin,synthesized in thyroid gland & transported into the follicular lumen (colloid). |
| Tyrosine residues on TG are iodinated to form | monoiodotyrosine (MIT) & diiodotyrosine (DIT). |
| Thyroxine (T4) is formed from | DIT+DIT. |
| Triiodothyronine (T3) is formed from | MIT+DIT. |
| Endocytosis & proteolysis of TG from colloid release T3 & T4 into | follicular cells. |
| T3 & T4 hormones then transported out of the cells by | diffusion into circulation. |
| Thyroid Hormone Secretion,Thyrotropin-releasing hormone (TRH) stimulates pituitary secretion of | TSH. |
| TSH, a glycoprotein is synthesized & secreted by | thyrotropes in the adenohypophysis. |
| Negative feedback control mechanism | maintain plasma levels of the these hormones. |
| Serum thyroid hormones are | highly protein bound & only the unbound or free form hormone are active. |
| Thyroxine binding globulin (TBG)binds about 60pct of | T4,Other proteins also other protein that binds T3 & T4. |
| T4 to T3 ratio is 4:1, but | T3 is more active. |
| Metabolism of Thyroid Hormones,T4 is converted to T3 & both can be conjugated with | glucuronide & sulfate in the liver. |
| Conjugates are excreted in the | bile. |
| Through entrohepatic circulation , some T3 & T4 are | liberated & reabsorbed from the GI tract. |
| Plasma half life: Human T4 & T3 = 7days & 1 day respectively,Dog | T4 = 12 to 24 hours & T3 about 8hrs. Cat T4= 11 hours. |
| Thyroid Hormones Preparations,Crude thyroid extracts,Extracts of thyroid tissues from animal (cow, sheep, pig) are available in the forms of | desiccated thyroid ( thyroglobulin). |
| Now we have suitable synthetic | T3 & T4 preparations available, thus there is no indication for use of crude extract. |
| Synthetic L-tri-iodothyronine | (tri-iodothyronine, liothyronine). |
| Synthetic levothyroxine is | T4 ( L-Thyroxine, thyroxine). |
| Synthetic levothyroxine (L-T4),T4 is the product of choice for the treatment | of hypothyroidism in all species,It is generally formulated & used as levothyroxine sodium for oral administration. |
| Injectable forms are also available for intravenous use in cases of | myxedema (nonpitting edema) coma. |
| Several preparation as generic & non generic compounds are available,They are all | T4 compounds. |
| Replacement therapy should be evaluated, based on resolution | of clinical signs, plasma levels of T4 , T3 & TSH. |
| Synthetic L – Triiodothyronine,Although T3 is active intracellular hormone, there are few valid reasons to use this product for | replacement therapy & some good reasons not to use it. |
| It has short half life & requires | frequent dosing. |
| T4 may not be converted to T3, | adequately, but it has not been convincingly demonstrated in dog & cats. |
| With severe small intestinal malabsorption diseases of T4, T3 oral administration can be done, & its | very effective. |
| Adverse effects & drug interactions,Overdose, thyrotoxicosis, lack of binding protein, clinical sign of hyperthyroidism includes | nervousness, hyper-activity, weight loss, polyphagia, polydipsia, polyuria, panting, fever etc. |
| Drug interactions | Phenobarbital, glucocrticoids, sulfonamides, non-steroid anti-inflammatory drug etc. |
| Hypothyroid dogs with diabetes mellitus tend to be | insulin resistant & insuline requirements may decrease with thyroid hormone replacement. |
| Antithyroid Drugs,These drugs are indicated for the treatment of hyperthyroidism due to excessive secretion of thyroid hormones | Adenomatous thyroid hyperplasia,Functional thyroid adenocarcinoma. |
| Long term therapy for patients in which surgery or iodine – 131 treatment is | not possible. |
| Short term treatment before surgery to | reduce the risk. |
| Short term therapy to | assess the effect of correcting thyrotoxicosis on renal function. |
| Antithyroid Drugs,Goitrogens:With the isolation & purification of thyroid peroxidase (TPO) enzyme of the thyroid gl&, & with indication that there are compounds which can act by | being direct inhibitors of the thyroid peroxidase (TOP), leading to a reduction in the coupling step of thyroid hormone synthesis. |
| These drugs acts to inhibit the synthesis of thyroid hormones by following mechanisms | Block incorporation of iodine into the tyrosyl groups in thyroglobulin,preventing coupling of iodotyrosyl group ( MIT1 & DIT)This occurs by inhibition of enzyme TPO activity. |
| Antithyroid Drugs,Plants of the genus Brassia such as rutabaga, cabbage, turnip, contain a compound called goitrin | Which has antithyroid activity. |
| There are several other products has similar action & can cause | hypothyroidism. |
| There are also some environmental chemical which can cause | hyperthyroidism. |
| Thioureylene or Thionamides | These are antithyroid compound & used for treatment of hyperthyroidism. |
| Thioureylene or Thionamide drugs | Thiourea,Thiouracil,Propylthiouracil, due to adverse reaction it is not recommended for controlling thyrotoxicosis in cats,Goitrin,Methimazole(thiamazole),Carbimazol, a carbethoxy derivative of thiamazole. |
| Thioureylene antithyroid drugs,Mechanism of action | inhibit thyroid hormone synthesis by inhibiting thyroid peroxidase catalyzed reaction. |
| Also inhibits peripheral conversion of | T4 to T3, but this effect is limited to Propylthiouracil. |
| Thioureylene do not affect | the uptake of iodine or release of preformed hormone from the thyroid gland. |
| Methimazole (Thiamazole) | Is a preferred antithyroid medication for hyperthyroid cats. |
| Methimazole,Oral administration in cats, | Rapid absorption from GI track & bioavailability 80pct,No significant protein binding activity. |
| Metabolized by | hepatic cytochrome P450 enzyme, followed by conjugation with glucuronide in other species than cat. |
| Elimination half life in cats | 2-10 hours. |
| Significant reduction of signs of hyperthyroidism | is about 1 to 3 weeks. |
| Adverse effects | Anorexia, lethargy, pruritus, hepatopathy, hemorrhage, etc. |
| Transdermal Methimazole,Methimazole compounded in pluronic lecithin organogel (PLO) for | transdermal application. |
| Carbimazole | Available as 5 & 20 mg tablets for oral uses. |
| Carbimazole is a | carbethoxy derivative of methimazole & is rapidly metabolized to the parent compound, which is responsible for its antithyroid activity. |
| Adverse effects are | very similar to methimazole. |
| Carbimazole is | mostly used out side of North America. |
| Ipodate is an | iodine containing compound that inhibits the conversion of T4 to T3,May also block T3 receptors & action of TSH. |
| Sodium or calcium ipodate, a cholecystographic contrast agent that reduces thyroid hormone production, & has potential for the treatment of feline thyrotoxicosis,Can be | used if methimazole can not be tolerate. |
| Stable Iodine,Iodine is | not indicated for long term treatment in cats with hyperthyroidism, but has been used with beta- blockers for pre surgical treatments of cats unable to tolerate carbimazol or methimazole. |
| Large doses of stable iodine | acutely inhibit thyroid hormone synthesis & release,the effect of inhibition of thyroid hormone synthesis is mediated through in inhibition of the enzyme thyroid peroxidase (TPO). |
| Levothyroxine | Soloxine,Forthyron,Leventa, etc. |
| Methimazole(Thiamazole) | Tapazole, Felimazole. |
| Carbimazole | Neomercazole. |
| Iopanic Acid | Telepaque. |
| Calcium ipodate | Orgraffin. |
| Tri-iodothyronine | Tertroxin. |
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alljacks
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