Thyroid And Antithyroid Agents Flashcards by joseph dizon

Thyroid hormones essential for

Growth and development

Regulation of energy metabolism

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Synthesis of thyroid hormone dependent on

Dietary iodine 150-200 ug

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Major steps on synthesis of thyroid hormone

Iodine trapping
Oxidation of iodine 
Iodination of tyrosine
Formation of T3 and T4
Secretion 
Peripheral conversion of T4 to T3

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Ingested iodine is converted to iodide and absorbed in the GIT.
Principal organs involved

Thyroid - for hormone synthesis
Kidney- for secretion
Liver- for metabolism

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Is the active transport of iodide from circulation to colloid of thyroid gland
Iodide pump

Iodide trapping

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Iodide trapping is stimulated by

Thyrotropin

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Iodide trapping is inhibited by

Thiocyanate

Perchlorate

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Oxidation/Iodination

Iodide is oxidized to iodine by

Thyroid peroxidase

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Oxidation/Iodination

Results

Mono iodo tyrosyl

Diiodotyrosyl

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Formation of T3 and T4

Coupling reaction

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MIT + DIT = T3 or triiodothyronine

DIT + DIT = T4 thyroxine

T3 is 3-5x more active than T4

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T3 and T4 are synthesized and stored in the

Thyroglobulin

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Secretion of T3 and T4 is initiated by

Endocytosis

Exit the cell by exocytosis

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Peripheral conversion of T4 and T3

Normal daily production

T4- 70-90 ug

T3- 15-30 ug

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Enzyme involved in conversion of t4 to t3

5 deiodinase

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Transport of thyroid hormone in the blood

T3 and T4 bound in

Thyroxin binding globulin TBG

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Total t4 free

Total t3 free

0. 2-0.5%

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Only free form is active

T3 is less firmly bound

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Causes increase TBG with increase binding thus decrease in free form

Pregnancy and estrogen

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Half life

T3 - 2days or less

T4 - 6-7 days

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Half life in hyperthyroidism and hypothyroidism

Hyper- decreased to 3-4 days

Hypo- increased to 9-10 days

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Marine life sea fish / shells

200-1000 ug/kg

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5kg of veg/fruits
Or
3kgof meat or fresh water fish

100 ug iodine

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1 gram of iodized salt

100ug iodine

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2 forms of hyperthyroidism

Diffuse (grave dis. ) - dalagang malaking mata, may edema, bagsak ang resestensya. IgG Nodular toxic goiter (plummer's disease) - maganda na may goiter

Thyroid Hormone | Synthetic

Levothyroxine Liothyronin Liotrix

Thyroid Hormone Natural (pig)

Desiccated thyroid | Thyroglobulin

Preparation of choice for replacement therapy for hypothyroidism and suppression therapy

Levothyroxine

Levothyroxine Characteristics 1

Stability Content uniformity Low cost

Levothyroxine Characteristics 2

Lack of allergy Easy interpretation Long half life

Levothyroxine Preparation

Synthroid Levothroid Eltroxin - availe in the Philippines

Best used as a diagnostic test in T3 suppression and short term suppression of TSH

Liothyronine

Liothyronine Halos kabaliktaran ng levothyroxine in terms ng characteristics

Short half life (24hours)

More cardio toxic

Liothyronine

Liothyronine Preparation

Tertoxin, 20 ug/tab

A mixture of levothyroxine and iodothyromide in ratio of 4:1

Liotrix

Liotix preparation

Euthyroid | Thylolar

Fine powder from desiccated pig thyroid

Thyroid

Thyroid preparation

Armour | Thyran

Purified pig extract

Thyroglobulin

Thyroglobulin preparation

Proloid

Actions of thyroid hormone

``` Regualtion of growth and development Calorigenic effect Cardiovascular effect Metabolic effect Inhibition of secretion of TSH ```

Antithyroid agents 4 categories

Thionamides or thiourylenes Ionic inhibitors Iodide Radioactive iodine

Interfere directly with thyroid hormone synthesis

Thionamides or thiourylenes

Block the iodide transport mechanism

Ionic inhibitors

Suppression of thyroid hormone synthesis when in high doses

Iodine

Damages and destroy the gland by ionizing radiation

Radioactive iodine

Thionamides 2 groups

Propylthioracil PTU | Methimazole (tapazole)

Rapidly absorbed Peak serum level after 1 hour Mostly excreted in the kidneys as the inactive glucoronide within 24 hours

Propylthioracil

10x more active that PTU | Slower excretion than PTU

Methimazole

Completely absorbed and accumulate in the thyroid gland

Methimazole

Mother drug, converted to methimazole in vivo

Carbimazole

Preferred for pregnant women since it is strongly bound to protein

PTU

Thionamides Prevent hormone synthesis by inhibiting the

Thyroid peroxidasecatalyzed

Blocks coupling of iodo tyrosine but

Does not block iodide uptake by the gland

Thionamides Inhibit deiodinase

PUT but not methimazole

Thionamides Early toxicity

Maculopapular rash, fever

Thionamides Toxicity late

``` Vasculitis Hepatitis Arthralgia Lymphadenopathy Lupus like reaction Polyserositis ```

Thionamides Most dangerous toxicity

Agranulocytosis Sore with high fever are the signs

Blocks uptake of iodine by the gland thru competitive inhibition of iodide transport.

Anion inhibitors

Anion inhibitors Agents

Perchlorate Peterchnetate Thicyanate

Anion inhibitors May produce aplastic anemia

Perchlorate

Very toxic and use only for diagnostic purposes

Anion inhibitors

Anion inhibitors May even block organification of iodine Produce by plant glycoside like cabbage

Thiocyonate

Inhibit organification | Inhibits hormone release thru inhibition of thyroglobulin decreased the size and vascularity of the gland

Iodides

Iodides Giving >6mg/day rapid improvement in thyrotoxic symptoms within

2-7 days

Iodides Disadvantages

Delay onset of Thionamides therapy Prevent use of radioactive iodine therapy Cross placenta and cause fetal goiter

Iodides Should not be used alone because

The gland will escape from the iodide block in 2-8 weeks

Iodides Withdrawals

Thyrotoxicosis | Iodoenriched gland

Iodides Toxicity

Acneiform Metallic taste Anaphylaxis Conjunctivitis Rhinorrhea Drug fever Swollen salivary glands Mucous ulcer

Important alternative if iodide sa and Thionamides are contraindicated

Iodinated contrast media

Iodinated contrast media Agents

Ipodate | Lopanoic acid

Not approved of FDA

Iodinated contrast media

Only isotope used for thyrotoxicosis

Radioactive iodine

Radioactive iodine In solution as

NA 1311

Radioactive iodine

B rays 5 days half life 400-2000

Radioactive iodine Evidence of thyroid destruction

Epithelial swelling and necrosis Follicular disruption Edema Leukocytic infiltration

Radioactive iodine Advantages

Easy administration Effectiveness Low expense Absence of pain

Radioactive iodine Disadvantages

Genetic damage Leukemianeoplasia 35 yo and above

Radioactive iodine Contraindication

Pregnancy | Nursing mothers

Agents that deplete cathecolamines

Reserpine | Guanitidine

Useful because they impair tissue response at the receptor site

Beta blocker (propranolol)

Weight of thyroid gland

15-20 grams