22. Hypothyroidism

Question 1

Signalment?

Answer 1

Signalment:
Hypothyroidism Is rare in cats
but common in dogs

Question 2

Pathophysiology congenital and acquired?

Answer 2

Canine hypothyroidism;
Pathophysiology:
- Congenital: Congenital hypothyroidism Is rare
- Acquired hypothyroidism: Common and usually In dogs between 4- 10 years old (giant breeds can be younger):
> Primary acquired hypothyroidism:
Most common acquired form, 95% of cases
* Autoimmune destruction of thyroid gland, leading to lymphocytic thyroiditis resulting in impaired production and secretion of thyroid hormones
» Secondary acquired hypothyroidism
* Rare, only 5% cases
Deficiency of thyroid stimulating hormone (TSH) leading to thyroid follicular atrophy
* Caused by extrathyroid gland illness - neoplasia, systemic Illness, drug therapy and ·sick euthyroid syndrome
> Tertiary hypothyroidism: Very rare, due to reduced thyrotropin-releasing hormone (TRH)

Question 3

Sick euthyroid syndrome:

Answer 3

Sick euthyroid syndrome:
» Normal thyroid gland, but transient suppression of thyroid gland activity due to systemic Illnesses or drug administration (phenobarbitone, NSAIDs, corticosteroids)
» Results in low resting serum total T4, free T 4 will also reduce if illness is severe
> MUST treat the underlying illness then perform screening tests when animal has recovered

Question 4

Clinical signs?

Answer 4

Clinical signs:
- Hypothyroidism affects many body organs as it is required for normal cellular metabolic functions
- Clinical signs are gradual, subtle and vague:
- Lethargy
- Weakness and muscle wasting, weight gain without an increase In appetite, heat-seeking, corneal lipidosis
- Dull, brittle hair coat with alopecia that is bilateral symmetrical, overpressure points and tall,
hyperpigmentation, myxoedema tragic* facial expression
> Neurological signs: Weakness, ataxia, vestibular signs and facial paralysis

Question 5

Diagnosis?

Answer 5

Diagnosis:
- Commonly over-diagnosed, and require multiple tests and may need to delay testing If concurrent illness is present due to sick euthyroid syndrome·
- Treatment trial without diagnostics is NOT appropriate
- Drugs can lower T4 levels: Corticosteroids, phenobarbitone, trimethoprim sulfa
- Haematology and biochemistry:
» Non-specific changes, non-regenerative anaemia (normocytic and normochromic),
hypercholesterolaemia
- Endocrine testing:
> Overall low total T4, a low free T 4 by equilibrium dialysis with high TSH .. 98% specific for hypothyroidism
> Low total T4 and low free T4 by equilibrium dialysis with normal TSH can be due to non-thyroidal illness or drugs
- Serum total T4:
Low normal levels are suggestive of hypothyroidism but can be affected by sick euthyroid syndrome” and drugs
> Normal levels rule out hypothyroidism In 90% of cases
-Free T4:
> More specific than total T4
> If low <10nmoVL are consistent with hypothyroidism, strong indicator of hypothY!”oldlsm but can be
reduced with concurrent Illness, drugs and hyperadrenocortlclsm
> · Best if measured by modified equilibrium dialysis (MED)
- TSH (thyroid stimulating hormone):
> High TSH confirms hypothyroidism with a low total T4 or free T4 and with concurrent clinical signs
> Can be normal In 30% of dogs with hypothyroidism

Answer 6

Treatment:
- Lifelong therapy with synthetic T4 hormone
- Synthetic L-thyroxine:
> 0.02mg/kg PO BID- generic products are not recommended
- If concurrent diabetes, hypoadrenocorticism then monitor closely, as thyroid supplementation can lead to hyperglycaemia and ketoacidosis, and also hypoadrenal crisis.

Question 7

Monitoring?

Answer 7

Monitoring
- Re-evaluate after 6-8 weeks
- Response to therapy is the main indicator of treatment success:
- Should see improvements In clinical signs over six weeks- increased activity, weight loss, hair regrowth may take longer
- Assess serum total T4, 6 hours after administration of the hormone, should be within normal range if not increase dose ( If there has been no response to therapy with normal post-pill serum total T4 levels after 4-5 months, then re-evaluate diagnosis
Overdose of thyroid medication:
- Develop clinical signs of hyperthyroidism, stop medication, clinical signs abate over a few days, restart supplementation at reduced dose.