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SA Endocrine 2 (Thyroid) Flashcards

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1
Q

Iodine

A
  • Taken up by thyroid tissue (TSH dependent)

- Iodine –> iodide bound to tyrosine on the TG molecule by thyroid peroxidase

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2
Q

Thyroglobulin

A
  • precursor protein made by thyroid follicular cells
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3
Q

Thyroglobulin -

A

After iodine is bound to tyrosine on TG molecule by thyroid peroxidase, it is converted to T3 and T4

  • Only 20% T3 made in thyroid; 80% made from T4 in tissues as needed
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4
Q

T3 and T4 negative feedback

A
  • negative feedback on TRH in the hypothalamus and TSH in the pituitary
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5
Q

Thyroid hormone basics

A
  • Active in most cells in the body
  • Increases metabolic rate
  • Catabolic effects (muscle and adipose)
  • cardiac inotropic and chronotropic
  • Stimulates erythropoiesis
  • Regulates cholesterol synthesis/degradation
  • Normal growth of the neurologic/skeletal system (mental alertness and peripheral nerves in adults)
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6
Q

Thyroid hormone basics

A
  • Most thyroid hormone is protein bound (99%)
  • Only free form (non protein bound) is active and enters cells
  • Free T3 or T4
  • T3 is more biologically active than T4
  • Most T3 produced peripherally from T4
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7
Q

Primary hypothyroid

A
  • Potential antibody formation against thyroglobulins and/or thyroid hormones
  • Thyroid tissue destruction resulting in decreased T3 and T4 production
  • Loss of negative feedback to pituitary gland
  • Increase in TSH production (~70%)
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8
Q

Thyroid stimulating hormone (TSH) or Thyrotropin Alfa (thyrogen - rhTSH) Indication

A
  • Gold standard hormone used for diagnosis of hypothyroid

- Recombinant human product

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9
Q

MOA of TSH

A
  • Increases iodine uptake by thyroid glands and increases production/secretion of thyroid hormones
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10
Q

TSH stimulation use

A
  • Evaluates the thyroid gland’s ability to respond to TSH and produce thyroid hormones
  • Useful for differentiating hypothyroidism from non-thyroidal illness
  • In non-thyroidal illness you have decreased TSH
  • Normal thyroid gland, so in non-thyroidal illness it should decrease
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11
Q

Adverse effects of TSH

A
  • Hypersensitivity (human product)
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12
Q

Cost of TSH

A
  • VERY $$$$
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13
Q

Levothyroxine

A
  • T4
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14
Q

Levothyroxine indication

A
  • Treatment of choice for hypothyroidism
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15
Q

Route of Levothyroxine

A

PO

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16
Q

Dosage of Levothyroxine

A
  • dose is must higher than in people because dogs have a higher 1st pass metabolism
  • Human pharmacists may question our dose
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17
Q

Levothyroxine cautions

A
  • Hypoadrenocorticism, cardiac disease, diabetes mellitus

- Consider dose reduction

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18
Q

Adverse effects of levothyroxine

A
  • Start with 1/2 dose in dogs with heart disease as it will increase myocardial oxygen demand
  • Can develop hyperthyroidism
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19
Q

Drug interactions of Levothyroxine

A
  • Many! Check before using
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20
Q

Other considerations for levothyroxine

A
  • Food may decrease bioavailability
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21
Q

Liothyronine MOA

A
  • Direct hormone replacement of T3
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22
Q

Cautions of Liothyronine

A
  • Not recommended
  • More expensive than T4
  • Does not increase T4 concentrations
  • T3 is produce as needed by tissues so could overdose some tissues
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23
Q

Indications of Liothyronine

A
  • Again, in general, don’t use
  • Use when there is no response to T4 administration suspected due to poor GI absorption
  • T3 has better GI absorption than T4
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24
Q

Thyroid extracts/dessicated thyroid Source

A
  • Most often porcine (Armour thyroid)
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25
Thyroid extracts/dessicated thyroid
- Powdered thyroid glands collected at slaughterhouses - Contain T4 and T3 in 4:1 ratio - Less expensive than synthetic T4
26
Indications for thyroid extracts/dessicated thyroid
- Not recommended - Potential for allergies/sensitivities - Variability from batch to batch - Variation in shelf life - Difficulty in maintaining proper regulation with T4/T3 content
27
Hyperthyroid overview
- Usually benign nodule develops in thyroid gland (can be bilateral) - Thyroid nodule produces excessive amounts of T4 - Negative feedback suppresses TSH production - Normal thyroid tissue atrophies and stops producing thyroid hormones
28
Liothyronine for hyperthyroidism
- Administered as part of the T3 suppression test for hyperthyroidism - Should inhibit TSH production - If TSH decreases, then T4 will decrease - Following T3 administration, T4 should be <50% baseline in normal cats, but there will be minimal suppression in hyperthyroid cats - Usually takes about 3 days, have to give a pill over 3 days
29
Methimazole Indication
- Drug of choice for hyperthyroidism
30
MOA Methimazole
- Inhibits thyroid peroxidase (therefore inhibits iodide binding to tyrosine to form T3 and T4
31
Dosing Routes of Methimazole
- Oral tablets - good bioavailability - Transdermal (compounded) - Dose adjusted every 2-3 weeks as needed based on CBC/Chem/UA/Total T4 levels
32
Transdermal methimazole
- Great - Compounded - Must be in pluronic lecithin organogel (PLO) - Fewer GI side effects - May take longer to see maximum effects
33
Cautions with transdermal methimazole
- Can be transferred to people or other pets
34
Adverse effects of methimazole
- Most are generally reversible - GI upset (reduce dose, divide dose, switch to transdermal) - Neutropenia/thrombocytopenia in 3-9% (monitor CBC and discontinue the drug if needed) - Facial excoriation in 2-3% (pruritus and erythema and discontinue drug if needed) - Hepatotoxicity (monitor biochemistry panel/liver enzymes and discontinue drug if noted) - Renal decompensation (cats already have underlying renal disease, and hyperthyroidism will increase GFR. Treating will decrease GFR to unmask renal disease)
35
Drug interactions of methimazole
-- Needed doses of some medications may change once the cat has become euthyroid - Diabetic may need less insulin
36
How long does it take to suppress T4 concentration with Methimazole?
1-3 weeks
37
Carbimazole Indication
- Hyperthyroid
38
Carbimazole MOA
- Pro-drug converted to methimazole
39
Carbimazole adverse effects
- fewer GI effects
40
Carbimazole availability in the US
- Not available in the US
41
Stable iodine (iodide) or potassium iodate MOA
- Large amounts of iodide given over 1-2 weeks may inhibit organification of thyroid hormone - Wolff-Chaikoff effect
42
Indication of stable iodine
- Could be used for hyperthyroid - Only useful for short term control (1-3 weeks) - Will not achieve complete remission
43
Adverse effects of Stable iodine
- GI upset | - Tastes bad - give in gel cap
44
I-131 (radioactive iodine) indication
- Treatment of hyperthyroid
45
I-131 MOA
- Concentrated in hyperfunctional thyroid cells as they take up iodine to make thyroid hormone - Beta particles destroy the tissue that takes up I-131 - Function of the normal thyroid tissue is suppressed and not producing hormone so it is preserved
46
Other considerations of I-131
- Must be administered by trained personnel in designated facilities - Cats are quarantined after administration until radiation levels diminish (1 week to 3 weeks)
47
Amlodipine Indication with Hyperthyroid
- Hypertension
48
Beta blockers indication
- Sympathetic overdrive with hyperthyroid | - e.g. propanolol, atenolol
49
Diabetes insipidus (Central)
- Partial or complete deficiency of ADH production | - Congenital, neoplasia, trauma
50
Diabetes insipidus (Nephrogenic)
- Defect in nephron causing lack or impaired renal tubular responsiveness to ADH
51
Primary Nephrogenic Diabetes Insipidus
- Congenital
52
Secondary Nephrogenic Diabetes Insipidus
- Conditions affecting ADH binding and function (hyperadrenocorticism, neoplasia, glucocorticoids, hyperaldosteronism, pyometra, hypercalcemia, etc.)
53
Desmopressin acetate (DDAVP) MOA
- Synthetic analog of vasopressin (ADH) | - Replaces ADH
54
Indications for Desmopressin acetate (DDAVP)
- Used for treatment of Central DI | - Used for differentiation of central DI from nephrogenic DI
55
Route of DDAVP
- Nasal solution given as an eye drop of SC | - 1 Drop in the conjunctival sac that can increase up to 4 drops
56
Adverse effects of DDAVP
- Uncommon - ocular irritation if given conjunctivally
57
Oral Bioavailability of DDAVP
- Low
58
Cost of DDAVP
- $$$ Expensive
59
Concentrations of nasal solution of DDAVP
- Be sure to get correct concentration of nasal solution as several products are available
60
Thiazide diuretics (chlorothiazide, hydrochlorothiazide) MOA
- Work in the early distal tubule to inhibit Na/Cl cotransporter - Causes enhanced excretion of Na and water
61
Thiazide diuretics and DI
- Reduces clinical PU/PD - mechanism not well understood with several proposed theories (inhibit distal sodium resorption, volume contraction, increased proximal tubular sodium and water resorption)
62
Route for thiazide diuretics
- Oral
63
Drug interactions for thiazide diuretics
- Many! Check before giving

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