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Pharmacology 2 > Thyroid Drugs > Flashcards

Thyroid Drugs Flashcards

1
Q

What is T4?

A

Thyroxine

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2
Q

Where is T4 made?

A

100% in the thyroid

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3
Q

Where is T3 made?

A

20% in the thyroid, the rest is from T4

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4
Q

What percentage of T3 and T4 is “free”?

A

~1%

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5
Q

Which Thyroid hormone is the “biologically active” form?

A

T3

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6
Q

Where is T4 deiodinated to T3? (3 things)

A

Liver
Kidney
Muscles

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7
Q

Which thyroid hormone enters cells more quickly?

A

T3

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8
Q

Which thyroid hormone has the more rapid onset?

A

T3

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9
Q

Which thyroid hormone is more potent?

A

T3 is 3-5x more potent than T4

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10
Q

How do thyroid hormones work?

A

They bind to receptors in the nuclei and influence production of regulatory enzymes.

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11
Q

What usually causes hypothyroidism? (2 things)

A

Lymphocytic thyroiditis

Idiopathic atrophy

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12
Q

What is the goal of treating hypothyroidism?

A

Replacing the hormone

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13
Q

What are the two pharmacologic agents available for treating hypothyroidism?

A

Levothyroxine (T4)

Liothyronine (T3)

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14
Q

Which has faster onset of action, Liothyronine (T3) or Levothyroxine (T4)?

A

Liothyronine (T3)

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15
Q

Which has a shorter half-life, Liothyronine (T3) or Levothyroxine (T4)?

A

Liothyroxine (T3)

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16
Q

Which would you use in dogs with reduced ability to convert T4 to T3?

A

T3

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17
Q

Which has greater GI absorption in dogs between T3 and T4?

A

T3

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18
Q

Which is the treatment of choice in all species with hypothyroidism and why?

A

T4, because it allows for cellular autoregulation providing T3 as the cell needs it. The longer half-life also makes dosing easier.

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19
Q

What can T3 be used to diagnose in cats?

A

Hyperthyroidism

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20
Q

How is levothyroxine usually given?

A

Oral tablet

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21
Q

When does levothyroxine reach peak plasma concentration?

A

4-12 hours

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22
Q

What is the half-life of levothyroxine?

A

12-16 hours

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23
Q

What is a caution with levothyroxine?

A

Acute overdose less of an issue than chronic

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24
Q

What can you see with thyrotoxicosis? (6 things)

A 
V&D
Tachycardia
Tachypnea
Weight loss
Hyperactivity
Hypertension
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25
Q

How do you monitor thyroid levels?

A

Plasma T4

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26
Q

How often do you monitor patient thyroid levels?

A

4 weeks after starting dosing

27
Q

What is the starting dose for Levothyroxine?

A

0.02mg/kg PO BID

28
Q

What is the alternate dosing for dogs >50-60lbs (~25kg)?

A

Body surface area (0.5mg/m^2)

29
Q

How long after stopping the drug can you see effects of Levothyroxine continue?

A

Up to 4 weeks

30
Q

What is a rare manifestation of hypothyroidism?

A

Myxedema coma

31
Q

In cases of Myxedema coma, what are 4 things that you do to treat?

A

INJECTABLE levothyroxine
Warm the patient
Respiratory support
Fluid/electrolyte support

32
Q

What two anticonvulsants don’t interfere with thyroid testing?

A

Levetiracetam

Potassium bromide

33
Q

How long can general anesthetics interfere with thyroid testing?

A

Up to 48 hours

34
Q

What can hyperthyroidism cause?

A

Thyrotoxicosis from excess thyroid hormone (T3 and T4)

35
Q

What is hyperthyroidism most commonly due to in cats?

A

Functional adenomatous hyperplasia

36
Q

What are two less common causes of hyperthyroidism in cats?

A

Thyroid adenoma

Thyroid carcinoma

37
Q

What is the cause of hyperthyroidism in dogs? (2 things)

A

Iatrogenic

Functional neoplasia

38
Q

What are the 3 main treatment options for hyperthyroidism?

A

Radioactive iodine (I131)
Antithyroid drugs
Diet (Y/D)

39
Q

What are two other treatments for hyperthyroidism?

A

Thyroidectomy

Intrathyroid injection of ethanol (don’t recommend lots of complications)

40
Q

What does the radioactive iodine treatment entail?

A

Single injection of I 131 to selectively destroy thyroid tissue

41
Q

How long is the half-life of I 131?

A

~8 days

42
Q

How long must the I 131 patient be in isolation after treatment?

A

1-4 weeks

43
Q

T/F: Radioactive I 131 is very safe.

A

True, but it’s very expensive.

44
Q

What 3 drugs fall under Thioureylenes?

A

Methimazole
Carbimazole
Propylthiouracil (PTU)

45
Q

How do thioureylenes work?

A

They inhibit synthesis of thyroid hormones by stopping organification and coupling

46
Q

T/F: Methimazole lowers T4, but T3 is normal.

A

True, due to the autoregulatory mechanisms in tissues.

47
Q

What species is Methimazole licensed for use in?

A

Felines

48
Q

How do you monitor Methimazole dosing?

A

By checking T4 levels after 2-4 weeks

49
Q

What are the two major side effects of Methimazole?

A

Vomiting, anorexia (~10%)

Transient, mild hematology changes (~15%)

50
Q

If a cat has an idiosyncratic reaction to Methimazole, can you give it another thioureylenes?

A

NO! Must give the cat something from another group.

51
Q

T/F: Transdermal methimazole prevents adverse effects?

A

False, it may reduce GI upset, but does not prevent other adverse effects or idiosyncratic reactions.

52
Q

How does Carbimazole work?

A

It’s converted to Methimazole, and then goes from there.

53
Q

What are the three types of iodides?

A

Lugol’s solution
Potassium iodide
Sodium iodide

54
Q

How do iodides work to treat hyperthyroidism?

A

Inhibit organification and preformed hormone secretion

55
Q

Are iodides considered a first line drug?

A

No

56
Q

What are the two iodinated radiographic contrast agents?

A

Ipodate

Iopanoic acid

57
Q

How do iodinated radiographic contrast agents help treat hyperthyroidism?

A

Inhibit conversion of T4 to T3 in tissues

58
Q

What do you have to monitor with iodinated radiographic contrast agents treatment?

A

Monitor T3

59
Q

Are iodinated radiographic contrast agents considered a first line drug?

A

No, effect questionable and may be transient.

60
Q

How does Y/D work?

A

Restricted iodine reduced T3/T4

61
Q

How do you monitor Y/D?

A

Monitor T4

62
Q

T/F: Hill’s Y/D has been shown to cause hypothyroidism.

A

False

63
Q

What 3 things must you consider when deciding how to treat?

A

Treat the patient, not the disease
Do no harm (weigh the risks)
Client concerns (administration, cost, availability)

Pharmacology 2 (9 decks)

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