Diagnosis and Treatment of Hypothyroidism in Yorkshire Terriers
Diagnostic Approach in Dogs
In Yorkshire terriers and other dogs, hypothyroidism is diagnosed by integrating clinical signs with thyroid hormone measurements, specifically serum thyroxine (T4) or free T4, combined with canine thyroid-stimulating hormone (TSH) levels. 1, 2
Clinical Presentation
- Affected dogs present with slow-onset decreased activity, cold intolerance, and characteristic skin changes including thickening and mild hyperpigmentation 2
- The hair coat becomes dry and coarse, with alopecia developing particularly in friction areas 2, 3
- Common signs include dryness, dullness, dandruff, scales, coarseness, and slow hair regrowth 3
- Non-specific laboratory findings may include mild anemia and hypercholesterolemia, though these do not occur consistently 2
Diagnostic Testing
- Measure serum T4 or free T4 (fT4) as the primary screening test, with values <1.5 μg/dL considered diagnostic for hypothyroidism 3
- Measure canine TSH concurrently, as the combination of low thyroid hormones with elevated TSH confirms the diagnosis 1, 4
- Thyroid hormone concentrations are influenced by systemic diseases and drugs, requiring interpretation in conjunction with clinical signs 4
- When diagnosis remains uncertain, perform a thyrotropin (TSH) stimulation test or thyroid gland biopsy for definitive confirmation 2
- Consider thyroglobulin autoantibodies to identify autoimmune etiology 1
- Advanced imaging (ultrasound or scintigraphy) can be used when diagnosis is not definite 4
Treatment in Dogs
Dogs with confirmed hypothyroidism require lifelong thyroxine (T4) replacement therapy at 30-50 mcg/kg/day divided into 2 doses. 2
Treatment Protocol
- Initiate T4 replacement therapy once diagnosis is confirmed through hormone assays and clinical correlation 2, 4
- Administer the total daily dose in 2 divided doses for optimal control 2
- Monitor clinical response within 2 months, with good response expected in approximately 40% of cases 3
- If clinical response is inadequate after 5-6 months, critically re-evaluate the diagnosis and consider co-existing endocrinopathies 4, 3
Special Considerations for Dogs
- When hypothyroidism coexists with hypoadrenocorticism (the most common concurrent endocrinopathy), steroids must be initiated before thyroid hormone to prevent adrenal crisis 4
- Doberman Pinschers, Great Danes, Poodles, Schnauzers, Irish Setters, and Boxers are overrepresented breeds 3
- Posttreatment evaluation should be based on peak serum T3 and T4 concentrations, clinical response, and resolution of presenting signs 3
Key Differences from Human Treatment
Diagnostic Differences
- Dogs require species-specific canine TSH assays, as human TSH assays are not applicable 1, 4
- In humans, TSH >10 mIU/L with normal free T4 defines subclinical hypothyroidism requiring treatment, whereas dogs are diagnosed based on absolute T4/fT4 values <1.5 μg/dL 5, 3
- Human diagnosis emphasizes TSH as the primary screening test with >98% sensitivity, while canine diagnosis relies more heavily on T4/fT4 measurements due to different reference ranges 5, 2
Treatment Differences
- Dogs require significantly higher weight-based dosing (30-50 mcg/kg/day) compared to humans (1.6 mcg/kg/day), reflecting species differences in thyroid hormone metabolism 5, 2
- Dogs receive divided doses twice daily, whereas humans typically take once-daily levothyroxine 5, 2
- Human treatment monitoring uses TSH every 6-8 weeks during titration with target TSH 0.5-4.5 mIU/L, while dogs are monitored by peak T3/T4 levels and clinical response 5, 3
- In humans, elderly patients or those with cardiac disease start at lower doses (25-50 mcg/day absolute dose), but this age-based dose reduction is not standard in canine medicine 5, 2