Veterinary Information:
Canine Hypothyroidism

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Differential Diagnoses of Canine Hypothyroidism:
-Hyperadrenocortisism (Cushing’s disease)
-Adipositas
-Feminisation Syndrome

Diagnosis:
1.)Clinical signs
2.)Blood Sample. Measure T4: reference [thyroxine]: 20-46 nmol/l
3.)If [T4] < 15 nmol/l, then also perform a [TSH] measurement in this blood sample.
4.)Decreased [T4] combined with an in increased [TSH] >0.6 µg/l is concurrent with the diagnosis canine hypothyroidism.
5.)Scintygraphy or biopsy.
Note 1: 80% of the cases with decreased plasma [T4] are caused by SES (Sick Euthyroid Syndrome). In this syndrome the decreased [T4] is not caused by a thyroid gland problem but by other diseases or by drugs. These can cause a decrease of the protein bound T4 plasma levels through protein-binding competition with T4. In SES the free T4 concentration is normal. Therefore it is important to also measure TSH blood levels in patients with a decreased T4.
Note 2: One third of the cases of canine hypothyroidism show a normal TSH plasma level. In cases with a decreased T4 combined with a normal TSH, scintygraphy or thyroid gland biopsy are necessary for a definitive diagnosis.

Treatment and Monitoring:
-Synthetic L-thyroxine which normalizes both T4 and T3 levels.
-The risk of causing an iatrogenic hyperthyroidism is low
-The initial dose of L-thyroxine: 0.02 mg/kg q 12 hours
-Dosage should be adjusted depending on monitoring results
-Improvement of activity can be seen after 1-2 weeks.
-Improvement of haircoat and bodyweight: 4-6 weeks.
-Skin hyperpigmentation, myocardial changes and neurological abnormalities can take several months to improve.

1.)Check the serum T4 level 6 weeks after starting the treatment: T4 should be within the reference range just before the time of administration of an L-thyroxine dose and slightly above reference range 6 hours after administration.
2.)Serum T4 should be checked every 6 weeks for the first 6 months of treatment.
3.)When normal T4 levels are reached with the treatment. It can be monitored every 6 months.
Note: Most dogs can be controlled with a maintenance dose of 0.2 mg/kg
L-thyroxin once a day.

Prognosis: There is an excellent prognosis for the clinical signs to return to normal once the right treatment dose is administered. Failure of treatment is mostly due to an incorrect diagnosis of hypothyroidism.

Literature: Ettinger SJ, Feldman EC, Textbook of Veterinary Internal Medicine, WB Saunders Company, Philadelphia (2000) 5th edition: 1419-1427

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