Prior to veterinary school, I worked as for a few years at a major medical center as a nurse in their endocrine clinic. In older human patients with hyperthyroidism, was not that uncommon for these folks to present with perfectly normal serum T4 values but have very high serum T3 concentrations.

Do cats also develop a similar syndrome of T3 hyperthyroidism? That is, can some hyperthyroid cats maintain normal serum concentrations of T4 and free T4, but have clinical signs resulting from high circulating T3 values alone?

I've reviewed the literature, but I don't see that T3 thyrotoxicosis has ever been reported in cats. Is that because we just aren't checking serum T3 levels in hyperthyroid cats? Do you think that we are missing these cases?

My Response:
This syndrome is commonly referred to as triiodothyronine (T3) thyrotoxicosis or just T3 toxicosis. This disorder is found almost exclusively in the elderly and may be the cause of hyperthyroidism in up to 10% of older patients. T3 thyrotoxicosis is characterized by the finding of high serum T3 with normal or even low T4 levels, which can sometimes even lead to an erroneous diagnosis of hypothyroidism.

In the late 1970s and throughout most of the 1980s, we routinely measured serum T3 on all hyperthyroid cats. Although serum T3 concentrations were very high in many of these cats, we never documented a cat with isolated T3 thyrotoxicosis (1).

So over the last 35 years, I have certainly looked for this syndrome of T3 thyrotoxicosis. I have always expected to find some cats with normal T4 but high T3 levels, but to date the syndrome of T3 hyperthyroidism has yet to be documented in cats.

Today, we rarely use serum T3 concentrations as a diagnostic test for hyperthyroidism. Use of total T4 alone is diagnostic in over 90% of cats, whereas serum T3 values are high in only about two-thirds of cats (2).

That said, however, it still is possible that hyperthyroid cats could develop T3 hyperthyroidism. Certainly in circumstances where a low iodine diet is fed, you might see a situation where T3 is being secreted preferentially over T4 (3).

In states of iodine deficiency, it’s easier for the thyroid gland to make T3 compared to T4. The reason for that is simple — it takes one less iodide atom to make T3 than T4, since T3 only has 3 iodine atoms whereas T4 contains 4 iodine atoms. T4 has 1 more iodine atom than T3


T4 has 1 more iodine atom than T3

Therefore, you have a cat in which hyperthyroidism is suspected, especially if a palpable goiter was present, but total and free T4 values are normal, I would certainly recommend that you run a serum T3 concentration to rule out T3 thyrotoxicosis.

References:


Peterson ME, Kintzer PP, Cavanagh PG, et al. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J Am Vet Med Assoc 1983;183:103-110.
Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. J Am Vet Med Assoc 2001;218:529-536.
Laurberg P. Mechanisms governing the relative proportions of thyroxine and 3,5,3'-triiodothyronine in thyroid secretion. Metabolism: clinical and experimental 1984;33:379-392.


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