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thyroid pruoit


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.
Created by: alljacks



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