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DEMS IV > thyroid pharmacology > Flashcards

thyroid pharmacology Flashcards

1
Q

Which drugs can cause hypothyroidism

A

lithium, amiodarone, cholestyramine, phenytoin, carbamazepine

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2
Q

LevothyroxineMOA

A

synthetic T4

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3
Q

Levothyroxine- time course

A

Resolution of symptoms begins within 2-3 weeks. Requires 6-8 weeks of maintenance dose to reach steady-state plasma levels . Thyroid function tests should be assessed at least 6-8 weeks after any dosage change

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4
Q

Levothyroxine- contraindications

A

cardiac disease

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5
Q

Hypothyroidism during pregnancy

A

Requires an increased dose (avg 25% increase) due to: 1. Increased levels of TBG (via  estrogen) decreases free T4-T3. 2. Increased placental metabolism of T4-T3.

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6
Q

Myxedema coma

A

An extreme form of hypothyroidism, so severe as to readily progress to death unless diagnosed promptly and treated vigorously. Na and glucose drop, hypothermia, shock and possibly death.

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7
Q

Myxedema coma treatment

A
  1. large doses of T4- IV loading dose followed by daily IV dosing. 2. hydrocortisone to prevent adrenal crisis b/c T4 may increase its metabolism
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8
Q

Thyroid hormones absorption

A

Best in ileum-colon. bioavailability - T4: 65-85%, T3: 95%. May be modified by binding proteins (T4), food or intestinal flora. Absorption is impaired in severe myxedema, so IV only.

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9
Q

Levothyroxine- instructions for usage

A

empty stomach with water, 30-60 min before breakfast or 4 hours after last meal in evening

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10
Q

Levothyroxine- drugs that can impair absorption

A

•Metal ions (antacids, calcium and iron supplements). Ciprofloxacin, bile acid sequestrants, raloxifene, sucralfate. Avoid interaction by spacing levothyroxine dose 2 hrs before or 4-6 hrs after interacting drug

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11
Q

amount of free T3 and T4 in plasma

A

free T4 = 0.04%, free T3 = 0.4%

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12
Q

Levothyroxine- drugs that affect protein binding

A

Increased binding: estrogens/SERMs, methadone, 5-fluorouracil, heroin. Decreased binding: glucocorticoids, androgens, salicylates, anticonvulsants (phenytoin-carbamazepine), furosemide

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13
Q

Activation of thyroid hormones

A

•T3 (80%) utilized by peripheral tissues is derived from T4 deiodination in liver via 5’-deiodinase. T3 in brain and pituitary derived by intracellular deiodination

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14
Q

Drugs that inhibit conversion of T4 to T3

A

glucocorticoids, Beta blockers, amiodarone, propylthiouracil

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15
Q

Conditions which inhibit conversion of T4 to T3

A

acute/chronic illness, caloric deprivation, malnutrition, fetal/neonatal period

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16
Q

Inactivation of T3

A

•Deiodination to reverse T3, Deamination, decarboxylation, conjugation to glucuronide or sulfate

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17
Q

Conditions which cause increased/decreased metabolic clearance of thyroid hormones

A

•Increased in hyperthyroidism and CYP450 induction - decreased by hypothyroidism

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18
Q

Half life of T3 and T4

A

T4: 7 days (due to protein binding). T3: 1 day.

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19
Q

When should Thyroid function tests be monitored for hypothyroidism

A

6-8 weeks after any change in levothyroxine

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20
Q

Liothyronine MOA

A

aka triiodothyronine. Synthetic T3.

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21
Q

Liothyronine uses

A

NOT recommended for routine replacement due to short t1/2 (greater Cp fluctuations between doses), high cost.

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22
Q

Liothyronine contraindications

A

cardiac disease- T3 activity has greater risk of cardiotoxicity. Also may increase risk of osteoporosis

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23
Q

Liotrix - MOA

A

4:1 mixture of T4 and T3.

24
Q

Liotrix uses

A

No advantage b/c T4 conversion to T3 in periphery results in near normal ratio. Rarely required, not recommended.

25
Q

Liotrix adverse outcomes

A

May increase incidence of low TSH concentrations and increase markers of bone turnover

26
Q

Thyroid USP MOA

A

Dessicated porcine thyroid extract containing T3 and T4

27
Q

Thyroid USP disadvantages

A
  1. Variable T4/T3 ratio and content - unexpected toxicities. 2. protein antigenicity. 3. instability.
28
Q

Thyroid USP uses

A

Less desirable than levothyroxine - current recommendation is use in hypothyroidism should be avoided

29
Q

Adverse reactions of thyroid hormones in children and adults

A

Children: restlessness, insomnia, accelerated bone maturation. Adults: anxiety, heat intolerance, palpitations-tachycardia, tremors, weight loss, diarrhea. Also sympathetic overactivity: can precipitate arrhythmias, angina, or MI in patients with cardiac dz

30
Q

Drug interactions with thyroid hormones

A

•Increased adrenergic effect of sympathomimetics: epi or decongestants (pseudoephedrine - phenylephrine)

31
Q

In general, treatment of graves disease

A
  1. interfere with hormone production (synthesis inhibitors): thionamides, idodides. 2. Modify tissue response (symptomatic improval): beta blockers, corticosteroids. 3. glandular destruction: radioactive iodine or surgery
32
Q

List thionamides

A

Methimazole, propylthiouracil

33
Q

Who responds best to methimazole- PTU

A

mild dz, small gland, young patients

34
Q

Benefits of specific beta blockers in graves disease

A

Used for symptom relief until hyperthyroidism is resolved. Propranolol: blocks T4 to T3 conversion. Metoprolol-atenolol: B1 selective, longer T1/2.

35
Q

Methimazole-PTU MOA

A

Inhibits thyroid peroxidase blocking T4/T3 synthesis. Blocks iodine organification and iodotyrosine coupling. At high doses PTU blocks peripheral conversion of T4 to T3.

36
Q

Time course of methimazole- PTU actions

A

requires 3-4 weeks to deplete T4

37
Q

Methimazole- PTU uses

A

Only for thyrotoxicosis from excess production (Graves disease - high RAI) NOT excess release (low RAI)

38
Q

methimazole- PTU absorption

A

rapid. PTU is incomplete. Methimazole is complete

39
Q

Methimazole- PTU distribution

A

Both can cross placenta and are concentrated by fetal thyroid, so use with caution in pregnancy. PTU is more protein bound so it crosses the placenta less readily and has less secretion into breast milk.

40
Q

Methimazole-PTU elimination/ half lives

A

Short half lives (PTU 1-2 hrs, methimazole 5-13 hrs) - but drugs accumulate in thyroid - thus clinical actions longer. PTU given 2-3 times per day. Methimazole once daily

41
Q

Methimazole-PTU uses

A

Clinical resolution of thyrotoxicosis within 2 weeks. Biochemical resolution in about 6 weeks.

42
Q

Situations where methimazole-PTU alone are effective in graves

A

small goiter, low level of anti-TSH receptor Ab, and mild-to-moderate hyperthyroidism

43
Q

remission with methimazole-PTU for graves

A

•Remission within 12-18 months. 1/3 will have lasting remission. 60-70% will have recurrence of graves

44
Q

Methimazol -PTU adverse rxns

A

Agranulocytosis is most dangerous. Pruritic rash, GI intolerance, arthralgias more common. For PTU only, hepatotoxicity is rare but serious.

45
Q

Compare methimazole vs PTU

A

Methimazole generally preferred: efficacy at lower doses, once-daily dosing, and lower side effect incidence. PTU is safer to fetus - treatment of choice in pregnancy

46
Q

list iodides

A

SSKI (super saturated potassium iodide) and Lugols solution (potassium iodide/iodine)

47
Q

SSKI- Lugols solution MOA

A

•Inhibit T4-T3 synthesis (via elevated intracellular [I-]). Inhibit T4-T3 release (via elevated plasma [I-]) > block Tg proteolysis

48
Q

SSKI- Lugols uses

A
  1. severe thyrotoxicosis-thyroid storm b/c rapid onset. 2. decrease size and vascularity of hyperplastic gland before surgery.
49
Q

SSKI- Lugols disadvantages

A

variable effects, rapid reversal when withdrawn, potential to produce new T3 and worsen hyperthryoidism

50
Q

SSKI- Lugols adverse rxns

A

acneform rash, rhinorrhea, metallic taste - swollen salivary glands (selective accumulation)

51
Q

Radioactive iodine MOA

A

Administered orally- concentrates in thyroid. Beta radiation causes slow inflammatory process that destroys the parenchyma of gland over a period of weeks to months.

52
Q

Radioactive iodine advantages

A

easy administration, effective, low expense, no pain

53
Q

Radioactive iodine disadvantages

A

slow onset (2-6 months and 10% require second dose), radiation thyroiditis via release of preformed T3 may cause cardio complications in elderly, worsening of ophthlamopathy, causes hypothyroidism.

54
Q

Who should not receive radioactive iodine

A

pregnant or nursing women, elderly

55
Q

Use of surgery for graves dz

A

Less commonly used b/c radioactive iodine has better benefit to risk ratio. 50-60% require thyroid supplementation afterwards due to iatrogenic hypothyroidism. Can be used in 2nd trimester of pregnancy if needed

56
Q

What is a thyroid storm

A

Sudden acute exacerbation of thyrotoxicosis causing fever, flushing, sweating, tachycardia-atrial fibrillation, delirium, coma. May occur if patients are non-compliant, incompletely treted, undiagnosed and have an acute stressor. Sx are due to hypermetabolism and excessive adrenergic activity

57
Q

Treatment of thyroid storm

A
  1. Propranolol- IV or PO to control CVS sx plus block T4 to T3 conversion. 2. NaI (IV) or KI (oral) to slow hormone release. 3. PTU blocks hormone synthesis plus blocks T4 to T3 conversion. 4. Hydrocortisone protects against shock plus blocks T4 to T3 conversion plus modulates immune response

DEMS IV (26 decks)

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