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DEMS: Unit III > Thyroid Pharmacology > Flashcards

Thyroid Pharmacology Flashcards

1
Q

Drugs leading to Primary hypothyroidism

A 
lithium, 
amiodarone*
cholestyramine, 
phenytoin,
 carbamazepine
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2
Q

Treatment of Hypothyroidism

A

Replacement therapy: Levothyroxine (T4) children get highest dose, seniors get lowest dose.

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3
Q

Tx of hypothyroidism resolves Sx when?

A

Resolution of symptoms begins within 2-3 weeks

Requires 6-8 weeks of maintenance dose to reach steady-state plasma levels

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4
Q

Contraindication of hypo Tx

A

heart disease (use caution)

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5
Q

Hypothyroidism Treatment - Pregnancy considerations

A

Usually requires increase in dose due to:

  • Increased levels of TBG (via  estrogen) decreases free T4-T3 - no intact gland to increase production
  • Increased placental metabolism of T4-T3
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6
Q

Myxedema coma: derangement and Tx

A

Acute medical emergency (low Na+, low glucose, hypothermia, shock, death)

  • Large doses of T4 required: IV loading dose followed by daily IV dosing
  • Hydrocortisone to prevent adrenal crisis as T4 may increase its metabolism
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7
Q

Levothyroxine pharmacokinetics

A

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

Drugs that can impair absorption of levothyroxine include:

  • Metal ions
  • bile acid sequestrants,
  • Avoid interaction by spacing levothyroxine dose 2 hrs before or 4-6 hrs after interacting drug
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8
Q

T of F: Changes in TBG levels or binding affinity will affect TOTAL serum levels

A

True, - NOT FREE

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9
Q

Drugs increasing thyroid hormone binding

A

Estrogens / SERMs

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10
Q

Drugs decreasing thyroid hormone binding

A

Anticonvulsants (phenytoin-carbamazepine)

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11
Q

Drugs that inhibit thyroid hormone activation

A

Glucocorticoids
Beta-blocke
Amiodarone
Propylthiouracil (higher doses)

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12
Q

Thyroid hormone Metabolic clearance rates

A
  • Increased in hyperthyroidism and CYP450 induction - decreased by hypothyroidism
  • Half-life of T4 = 7 days & Half-life of T3 = 1 day
  • Degree of protein binding major factor for difference
  • Long t1/2 T4: once-daily dose - Cp fluctuation 10%
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13
Q

Advantages of levothyroxine

A
  • Stability, content uniformity, lack of allergenic protein (vs Thyroid USP)
  • Low cost
  • Once-daily dosing – minimal fluctuation
  • Can be given orally or IV
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14
Q

Liothyronine

A
  • synthetic T3
  • Well absorbed, rapid action, but shorter duration of effect that permits quicker dosage adjustments (at 1-2 weeks intervals)
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15
Q

Liothyronine contraindications

A
  • NOT recommended for routine replacement due to short t1/2 (greater Cp fluctuations between doses), high cost
  • Should be avoided in patients with cardiac disease (increased T3 activity  greater risk of cardiotoxicity)
  • May increase risk of osteoporosis
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16
Q

Liotrix

A

4: 1 mixture of T4 and T3
- No advantage since T4 conversion to T3 in periphery results in near normal ratio
- More expensive
- Rarely required, not recommended
- May increase incidence of low TSH concentrations and increase markers of bone turnover

17
Q

Thyroid USP

A
  • Dessicated porcine thyroid extract containing T3 and T4

- Absorption characteristics and half-lives of T4 and T3 are same as in non-combination products

18
Q

Thyroid USP disadvantages

A

Variable T4/T3 ratio and content unexpected toxicities
Protein antigenicity
Product instability
Less desirable than levothyroxine - current recommendation is use in hypothyroidism should be avoided

19
Q

Thyroid hormone replacement ARS and DDI

A

-Toxicity: directly related to plasma hormone level equivalent to signs and symptoms of hyperthyroidism
Drug interaction with thyroid hormones
-Increased adrenergic effect of sympathomimetics: epi or decongestants (pseudoephedrine - phenylephrine)

20
Q

Tx of Graves

A

Medications
-Antithyroid drugs (methimazole, propylthiouracil)- Inhibit synthesis of thyroid hormone
-Beta blockers- Reduce systemic hyperadrenergic symptoms and effects (primarily tremor, palpitations, etc.)
Radioactive Iodine (131I)
Surgery

21
Q

Thionamides:

A

Methimazole - PTU (propylthiouracil)

22
Q

Thionamides use in graves`

A

Methimazole until remission (1-15 yrs)
β-blocker for symptom relief until hyperthyroidism resolved
-Propranolol has advantage of blocking T4->T3 conversion
-Metoprolol-atenolol are beta1 selective, longer t1/2

23
Q

Thionamides MOA

A

Inhibit thyroid peroxidase: prevent T4 / T3 synthesis

24
Q

use of thionamides in hyperthyroidism

A

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

25
Q

Does Methimazole - PTU Both cross placenta and are concentrated by fetal thyroid?

A

yes, Requires caution if used in pregnancy. PTU less readily b/c protein bound.

26
Q

Methimazole - PTU Effective alone IF:

A

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

27
Q

Thionamides: Methimazole - PTU adverse effects

A
  • rash
  • Most dangerous is agranulocytosis (0.3-0.6% - obtain baseline CBC)
  • PTU: Hepatotoxicity is rare, but serious enough
28
Q

Thiionamides SUMMARY

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
29
Q

Iodide ion medications

A
  • SSKI [super-saturated potassium iodide]

- Lugol’s solution [potassium iodide / iodine]

30
Q

Iodide ion MOA

A

Complex action, transient effect of high doses (> 6 mg daily):
-Inhibit T4-T3 synthesis (via elevated intracellular [I-])
-Inhibit T4-T3 release (via elevated plasma [I-]): block Tg proteolysis
Rapid onset: used in severe thyrotoxicosis - thyroid storm

31
Q

SSKI and Lugol’s Solution use, and adverse reactions

A

-Decrease size and vascularity of hyperplastic gland prior to surgery
-Adverse Reactions
Reversible: acneform rash, rhinorrhea, metallic taste - swollen salivary glands (selective accumulation)
-Potential to produce new T3  worsen hyperthyroidism

32
Q

Radioactive Iodine (131I) MOA

A

concentrates in thyroid

β-radiation causes slow inflammatory process that destroys parenchyma of gland over a period of weeks to months

33
Q

Radioactive Iodine (131I) disadvantages

A
  • Slow onset and time to peak effect (2-6 months to euthyroid state)
  • Radiation thyroiditis via release of preformed T3: cardiovascular complications in elderly
  • May cause worsening of opthalmopathy
  • Major complication is hypothyroidism
  • Should not administer to pregnant or nursing women

NO radiation-induced genetic damage, leukemia or neoplasia

34
Q

Surgery in hyperthyroidism

A

Less commonly used today as 131I has much greater benefit:risk ratio
Requires treatment with antithyroid drugs-iodide prior to surgery
iatrogenic hypothyroidism
Can be utilized in 2nd trimester of pregnancy if needed

35
Q

Thyroid Storm Sx

A

fever, flushing, sweating, tachycardia-atrial fibrillation, delrium, coma
-Caused by acute exacerbation of thyrotoxicosis (non-complient patient
Symptoms reflect hypermetabolism and excessive adrenergic activity

36
Q

Tx of thyroid storm

A
  • Propranolol - controls CVS symptoms plus blocks T4->T3
  • Hormone release slowed by NaI IV - KI drops orally
  • Hormone synthesis blocked by PTU plus block of T4->T3
  • Hydrocortisone protects against shock plus blocks T4->T3 - plus modulates immune response

DEMS: Unit III (21 decks)

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