Fluctuation in Serum Thyroid Hormone Concentrations: An Issue in Diagnosis of Feline Hyperthyroidism?

Examining a cat with hyperthyroidism

I am writing this email to you because of your recent article published in the Journal of Feline Medicine and Surgery about diagnostic testing for hyperthyroidism (1). In that paper, you mentioned that hyperthyroid cats can show fluctuation of serum T4 and T3 concentrations (2), leading to difficulty in diagnosis in some cats.

My question is this: When, in your opinion or experience, is the best moment of the day (or the best conditions), to get the blood samples for thyroid hormone analysis to avoid this fluctuation problem?

Thanks! Looking forward to hearing from you.

My Response:

Unfortunately, there is no way to predict if and when this fluctuation in T4 and T3 will occur. So collecting blood samples at a particular time of day would not be helpful.

Remember, however, that this fluctuation is only of clinical importance in cats with very mild hyperthyroidism; in these cats, the serum T4 value may vary from the upper third of the reference range limits to just slightly above normal. In cats with more advanced stages of hyperthyroidism, fluctuation in circulating T4 concentration will occur throughout the day and over a period of a few days, but all T4 values will be high (1-3). Thus, in most hyperthyroid cats, the fluctuation in circulating thyroid hormone levels will not contribute to problems in the diagnosis.

References:

1. Peterson ME. Diagnostic testing for hyperthyroidism in cats: more than just T4. J Fel Med Surg 2013;15:765-777.  
2. Peterson ME, Graves TK, Cavanagh I. Serum thyroid hormone concentrations fluctuate in cats with hyperthyroidism. J Vet Intern Med 1987;1:142-146. 
3. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: Hyperthyroidism. Compend Contin Educ Vet 2013;35:E1-E6.