Hypothyroidism

Question 1

Physiologic effects of thyroid hormones

Answer 1

Increase metabolic rate
Catabolic effects on muscle and adipose tissue
Cardiac - increased contractility and HR
Stimulates erythropoiesis
Regulates cholesterol synthesis/degradation
Normal growth of neuro/skeletal system (mental alertness, peripheral nerves in adults)

Question 2

What is the difference between primary and secondary hypothyroidism?

Answer 2

Primary - decrease in T3 and T4; common
Secondary - decrease in TSH; uncommon

Question 3

Three causes of primary hypothyroidism

Answer 3

Decrease in T3 and T4

1. Lymphocytic destruction (“thyroiditis”)
2. Idiopathic atrophy
3. Bilateral neoplasia

Question 4

Signalment for hypothyroidism

Answer 4

Usually middle aged dogs
Some breeds affected more often

Question 5

Common clinical signs of hypothyroidism

Answer 5

Metabolic

Dermatologic - symmetric alopecia, hyperpigmentation, otitis, seborrhea, poor coat quality, myxedema (causes sad face)

Neurologic - respond to levothyroxine; peripheral, cranial and CNS

Cardiovascular - bradycardia; decreased beta receptors

Reproductive dysfunction

Ocular - lipid issues

Megaesophagus, laryngeal paralysis

Question 6

Features of congenital hypothyroidism

Answer 6

Decreased cognitive ability
Stunted and disproportionate growth
Large, broad head with short, thick neck
Short limbs
Abdominal distention
Macroglossia
Hypothermia
Delayed dental eruption

Question 7

Hypothyroidism in cats is usually iatrogenic during:

Answer 7

Treatment for hyperthyroidism (I-131 or Methimazole)

Question 8

Should you test a dog for hypothyroidism who has no clinical signs of the disease?

Answer 8

NO

Question 9

A suspicion of hypothyroidism must initially be based off what three things?

Answer 9

History
Clinical signs
Physical exam findings

Question 10

Name two diseases that have overlapping clinical signs with hypothyroidism.

Answer 10

Atopic dermatitis
Hyperadrenocorticism (Cushing’s)

Question 11

Potential findings in a dog with hypothyroidism include:

Answer 11

• Normocytic, normochromic, nonregenerative anemia
• fasting hypertriglyceridemia
• fasting hyper cholesterolemia
• increased hepatic enzymes

Question 12

What is the gold standard diagnostic for hypothyroidism?

Answer 12

TSH-stim test
Very expensive to perform; may not be feasible

Question 13

Expectations for response to treatment of hypothyroidism

Answer 13

Clinical signs, improved activity 1-2 weeks
Weight loss 8 weeks
Neuro signs, hair coat several months

Question 14

Side effects of levothyroxine treatment for hypothyroidism

Answer 14

Hyperthyroidism - PUPD, polyphagia, tachycardia, nervousness

Lack of response/apparent tx failure
- inaccurate dx
- inadequate dose/compliance
- poor absorption (GI disorder)

Question 15

Treatment monitoring for hypothyroidism:

Recheck _____ four weeks after any dose change.
Once well regulated, check every ______.

Answer 15

T4
6 months

Question 16

Drug of choice for treating hypothyroidism

Answer 16

Levothyroxine

*half dose in cardiac patients (increases myocardial O2 demand)

*food may decrease oral bioavailability

Question 17

_______ is a good screening test and a good place to start with thyroid testing.

Answer 17

Total T4

*highly sensitive; low T4 will identify MOST hypothyroid patients; false positives common - normal variations

Question 18

How should you treat a dog with decreased total T4, normal to decreased free T4, and normal TSH?

Answer 18

This is a normal physiologic response and does not require supplementation.

During illness there is a decrease in thyroid response to TSH leading to activation of T4. TX is to correct the underlying illness.

Question 19

If you find a dog has low total T4, what is the next step before starting thyroid supplementation?

Answer 19

Additional testing:
Free T4
- active form; must be measured by Equilibrium Dialysis
- less affected by non-thyroidal illness

or

TSH
- will be high in animals with hypothyroidism without T3 and T4 being produced
- can be influenced by drugs, illness, pituitary exhaustion
- normal TSH does NOT rule out hypothyroidism

Question 20

A high ____ combined with a low _____ is diagnostic for hypothyroidism.

Answer 20

High TSH
Low T4

*normal TSH does NOT rule out hypothyroidism

Question 21

Presence of ______ antibodies is supportive but not diagnostic for hypothyroidism.

Answer 21

Anti-thyroglobulin
Anti-T3
Anti-T4