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DEMS: Unit III > Thyroid Pharmacology > Flashcards

Thyroid Pharmacology Flashcards

1
Q

Biosynthesis of thyroid hormones w/sites of antithyroid drug action (image)

A
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2
Q

Characteristics of thyroid hormone biosynthesis

A
  1. uptake of iodine
  2. iodide organification
  3. coupling of TH precursors
  4. storage and release

simple overview of thyroid hormone biosynthesis: http://www.handwrittentutorials.com/videos.php?id=17

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3
Q

Characteristics of iodine uptake in thyroid hormone biosynthesis

A
  • The major regulated step in the biosynthesis of TH is the uptake of I- (iodide ion) into the thyroid gland.
    • stimulated by TSH via Gs
    • blocked by anions of similar size such as SCN-, ClO4-, and I- itself.
  • High concentrations > 6 mg of I- cause decreased expression of transporter.
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4
Q

Characteristics of iodide organification in thyroid hormone biosynthesis

A
  • Iodide organification:
  • I- is oxidized and incorporated into tyrosine residues on…
  • thyroglobulin molecules 
(mono-[MIT] and di-iodinated [DIT] tyrosine) via thyroid peroxidase.
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5
Q

Characteristics of coupling of TH precursors in thyroid hormone biosynthesis

A
  • Coupling of TH precursors occurs on TG in a T4 / T3 ratio of 5:1. This process is also mediated via thyroid 
peroxidase.
  • MIT + DIT = T3 and
  • DIT + DIT = T4
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6
Q

Characteristics of storage and release of thyroid hormones

A
  • TH is retrieved from storage in the lumen by pinocytosis
  • slowly released from gland by proteolysis in a T4:T3 ratio of 20:1.
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7
Q

Advantages and disadvantages of levothyroxine

A
  • Levothyroxine = Synthetic T4
    • (Treatment of choice)
  • Adv:
    • Stability, content uniformity, lack of allergenic foreign protein compared to Thyroid USP, low cost, long half-life compared to T3 allowing once-daily dosing, oral or IV
  • Disadv:
    • Narrow therapeutic index
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8
Q

Advantages and disadvantages of liothyronine

A
  • Liothyronine = Synthetic T3
  • Adv:
    • Reasonable to add if symptoms persist on optimal levothyroxine therapy. Not generally recommended as long as deiodinase is functioning properly to convert T4 to T3
  • Disadv:
    • Short half-life, higher risk of cardiotoxicity, high cost, increases risk for osteoporosis
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9
Q

Advantages and Disadvantages of Liotrix

A
  • Liotrix = 4:1 Mixture of T4 and T3
  • Adv
    • Rarely required, not recommended
  • Disadv
    • Expensive, no advantage since T4 conversion to T3 in periphery results in near normal ratio, may cause increased incidence of low TSH and increased markers of bone turnover.
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10
Q

Advantages and Disadvantages of thyroid USP

A
  • Thyroid USP = Thyroid extract from pigs
  • Adv
    • Less desirable than levothyroxine, but still suitable for clinical purposes.
  • Disadv
    • Variable T3/T4 ratio can cause unexpected toxicities, allergenic foreign protein can cause protein antigenicity, product instability
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11
Q

Thionamides: MOA

A
  • Prevents T3/T4 synthesis by blocking iodine organification and coupling of iodotyrosines.
  • At high doses prevents conversion of T4 to T3 in peripheral tissues.
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12
Q

Thionamides: Pharmacokinetics

A
  • Rapid absorption, crosses the placenta and concentrated by fetal thyroid, relatively short half-life.
  • Methimaxole is generally prefered because of efficacy at lower doses and lower side effect incidence.
  • Propylthiouracil (PTU) may be better in pregnancy because it crosses the placenta less readily than Methimaxole.
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13
Q

Thionamides: toxicities

A
  • Pruritic rash
  • gastric intolerance
  • arthralgias
  • agranulocytosis
  • hepatotoxicity
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14
Q

Iodides: MOA

A
  • At high doses they inhibit hormone synthesis via elevated intracellular iodide,
  • inhibit hormone release through inhibition of thyroglobulin proteolysis via elevated plasma iodide.
  • Inhibits conversion of T4→T3
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15
Q

Iodides: Pharmacokinetics

A
  • Rapid effect, some patients show no response
  • rapid reversal of inhibitory effect when withdrawn
  • potential that the iodide may be used to produce new thyroid hormone and worsen hyperthyroidism
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16
Q

Iodides: toxicities

A
  • Acneiform rash, rhinorrhea, metallic taste, swollen salivary glands.
  • Toxicity is uncommon and signs/ symptoms of toxicity are reversible.
  • Gland “escapes” iodide block in 2-8 weeks
17
Q

Radioactive iodine (131I): MOA

A
  • Concentrated in the thyroid.
  • β radiation causes a slow inflammatory process that destroys parenchyma of gland over weeks to months
  • major complication is hypothyroidism
18
Q

Radioactive iodine (131I): Pharmacokinetics

A

Rapidly absorbed but slow onset and time to peak effect

19
Q

Radioactive iodine (131I): Toxicities

A
  • Radiation thyroiditis = release of preformed thyroid hormone and can result in cardiovascular complications in elderly or susceptible patients
  • may cause worsening opthalmopathy, hypothyroidism
  • do not administer to pregnant or nursing women
20
Q

Treatment of Graves disease

A
  • Interfering with hormone production → thionamides, iodides
  • Modifying tissue response to improve symptoms → β-blockers, corticosteroids
  • Glandular destruction → surgery, radioactive iodine
21
Q

Characteristics of thyroid storm

A
  • Thyroid Storm is a sudden, acute exacerbation of thyrotoxicosis
  • may occur in a non-compliant, incomplete treated, or undiagnosed patient with hyperthyroidism who experiences an acute stress such as infection, surgery, or trauma
22
Q

Treatment of thyroid storm

A
  • Propranolol controls cardiovascular manifestations and blocks conversion of T4 to T3
  • Sodium iodide given IV or potassium iodide given as oral drops can be used to slow the release of hormones.
  • Propylthiouracil (PTU) can be used to block the synthesis of hormones and prevent the conversion of T4 to T3
  • Hydrocortisone is used to protect against shock, block conversion of T4 to T3, and may modulate the immune response that leads to exacerbation of thyrotoxicosis.
23
Q

Antithyroid drugs:

Risk of relapse following Tx

Safety in pregnancy

Risk of hypothyroidism and need for hormone replacement following Tx

Typical age group

Type of disease

A

High

Thionamides are treatment of choice in pregnancy. Propylthiouracil (PTU) is best.

Low

Young patients

Mild disease

24
Q

Radioactive Iodine (131I):

Risk of relapse following Tx

Safety in pregnancy

Risk of hypothyroidism and need for hormone replacement following Tx

Typical age group

A

Low

Contraindicated in Pregnancy

80%; Greatest risk in children

Most often patients > 21 but becoming more common in patients ≤ 20

25
Q

Subtotal thyroidectomy:

Risk of relapse following Tx

Safety in pregnancy

Risk of hypothyroidism and need for hormone replacement following Tx

Typical age group

Type of disease

A

Low

Can be utilized in second trimester if needed.

50-60%

All ages

Tx of choice if large gland; Rarely used today because radioactive iodine works so well.

26
Q
A

DEMS: Unit III (24 decks)

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