Monitoring & Control

Hormonal goals

Peak tT4 should ideally be 35 to 95 nmol/l (2.7 to 7.4 µg/dl) - with values ideally in the upper half of the reference range for normal dogs (around 20-50 nmol/l or 1.6 to 3.9 µg/dl). Values between 50 nmol/l and 95 nmol/l (3.9 to 7.4 µg/dl) are usually associated without signs of thyrotoxicosis.

TSH concentration should be <0.68 ng/ml (within the reference range for normal dogs). Further assessment of hormonal responses and dose adjustment may be repeated at 4 week intervals if required.

Total T4 measurement cannot be relied on if T4 autoantibodies are present. Measure free T4 by equilibrium dialysis instead.2 The presence of autoantibodies to T4 will not affect the response to thyroid hormone replacement therapy.

Adjusting the Dose

For information on how to adjust the Leventa dose click here.

Many hypothyroid dogs will initially lose weight when starting on a replacement therapy. It may be necessary to decrease the dose of Leventa based on the new body weight. By monitoring the serum T4 and TSH concentrations 4 weeks after starting a dog on Leventa or after any dose change, adjustments to the dose of Leventa may be made accordingly.

Thyrotoxicosis

Thyrotoxicosis is uncommon in the management of canine hypothyroidism. Thyrotoxicosis can occur if a dog receives an excessive overdose and/or has impaired clearance of thyroid hormone. Chronic oversupplementaion with levothyroxine sodium is generally more likely to cause thyrotoxicosis than an acute overdosage.³

Clinical signs of overdose are identical to those of hyperthyroidism

• body weight loss
• hyperactivity
• tachycardia
• polyuria and polydipsia
• polyphagia
• diarrhoea.

Signs are generally mild and fully reversible on treatment cessation. Overdose may be accompanied by reversible changes in blood biochemistry, e.g. elevated glucose, inorganic phosphorus and albumin:globulin ratio, and reduced total protein and cholesterol. If overdose is suspected, standard measures should be taken to remove non-absorbed drug from the gastro-intestinal tract. If thyrotoxicosis is suspected, treatment with Leventa should be suspended until the signs of thyrotoxicosis resolve, which usually occurs within 1-3 days after discontinuing thyroid supplementation. If chronic overdosage is suspected, the dose should be re-evaluated and the serum tT4 and TSH concentrations measured 4 weeks later.

Long-Term Maintenance

A maintenance dose rate between 10 and 40 µg/kg body weight once daily is generally sufficient to control the clinical signs of hypothyroidism and to restore thyroid hormone concentrations to within the reference range.

Once a suitable dose rate and regime have been established, it is recommended to recheck every 6 months that tT4 and TSH concentrations 4-6 hours post-dosing are appropriate.

Clinical Response to Leventa Therapy

The improvement in clinical signs occurs differentially following the onset of treatment. Activity level may start to improve within 1 week. Metabolic signs start to improve within 2 weeks. Improvements of neurologic signs are usually seen in the first 1-2 weeks, but may take up to 3 months¹. Dermatological signs may require 6 weeks or more of treatment before improvement is seen. Healthy weight loss will likely be observed over the next few months as exercise naturally increases and improvements in fat metabolism occur.

References

1. Nelson RW & Feldman EC. (2004) Hypothyroidism. In: Canine and Feline Endocrinology and Reproduction. 3rd Edition. Saunders, St Louis, MO, USA.pp 86-142.
2. Schachter S, Nelson RW, Scott-Moncrieff JC, Ferguson DC, Montgomery T, Feldman EC, Neal L & Kass PH. (2004) Comparison of serum-free thyroxine concentrations determined by standard equilibrium dialysis, modified equilibrium dialysis, and 5 radioimmunoassays in dogs. J Vet Intern Med 18:259-264.
3. DeClementi Safrit C. (2001) Acute Thyroid Hormone Supplement Overdosage. Vet Med 96:424-430.

Dermatologic clinical signs common