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Hypothyroidism and Thyroid Testing in Dogs

  • August 12, 2016 11:04 AM EDT

    Hypothyroidism is not uncommonly misdiagnosed in dogs. This is owed, in part, to the insidious nature of the condition, the vagueness of clinical signs, and the challenge of interpreting results of various clinical tests in light of comorbid disease conditions or concurrent medication administration. 

    • Systemic diseases (nonthyroidal illnesses) that can alter (reduce) measured thyroid hormone concentrations include: adrenal disorders (hyper- and hypoadrenocorticism), diabetes ketoacidosis, kidney failure, liver disease, heart failure, critical illness, among several others.
    • Examples of drugs that can alter (reduce) measured thyroid hormone levels include sulfonamides and phenobarbital. There are several other classes of drugs known to have the same effect in humans, however evidence-based information on their effects in dogs is lacking.

    Many of our canine patients are a conundrum when it comes to making a definitive diagnosis of hypothyroidism. As such, therapeutic trials are commonly initiated by clinicians. These trials are often an attempt to "back into the diagnosis." A positive response to therapy is considered, by some, to support the diagnosis (e.g. reversal of common clinical signs including lethargy, weight gain, alopecia, seborrhea, etc.).  A positive response to thyroid hormone supplementation, however, should be interpreted with caution. Euthyroid dogs may also seem to improve with hormone supplementation.

    It is not uncommon for many of these dogs to continue on long-term supplementation. So, how does one determine if a dog on long-term treatment with levothyroxine (L-thyroxine) is truly hypothyroid (and needs ongoing therapy) or is euthyroid (and, by definition, no longer requires supplementation)?

    One way is to discontinue supplementation and observe for recurrence of clinical signs, along with monitoring of thyroid tests including serum T4, fT4, and TSH levels.  But how long is the "washout" period?  How long must a clinician wait after discontinuing supplementation with levothyroxine before performing these tests and accurately assessing thyroid function?  

    What's your answer? Is it:

    • 7 days?
    • 2 weeks?
    • 4 weeks?
    • 8 weeks?

    Until recently, veterinarians relied on studies in humans in which a period of 8 weeks is recommended. In this week's VetVine Specialty Update we report on important information recently presented at the 2016 ACVIM Forum - specifically, the effects of L-thyroxine supplementation on the hypothalamic-pituitary-thyroid axis in euthyroid dogs and how long it takes to recover after long-term supplementation. News you can truly use!

    View This VetVine Specialty Update (running time: 10 mins)