When To Start Thyroid Hormone Replacement in Cats Treated with Radioiodine (I-131)

I have a question about thyroid hormone supplementation for iatrogenic hypothyroidism, especially in cats treated with radioiodine (I-131). More specifically, how long after radioactive iodine therapy do you wait before recommending supplementing hypothyroid cats with thyroxine?

I work as a small animal internist at a referral hospital where we treat hyperthyroid cats with radioiodine. After treatment, we routinely run serum T4 and free T4 concentrations and full blood work 30 and 90 days after the cat is discharged. I have found that about 20% of these cats are biochemically hypothyroid (low total or free T4 values) at the 30-day recheck, but many of these cats will revert to normal by the 90-day recheck. The other internist at my practice supplements these cats with L-thyroxine at the first recheck if the serum T4 and free T4 values are low. She does this even if they are not azotemic, with the rationale being that the studies show that hypothyroid cats develop worsening azotemia, which can affect their survival (1).

I am not sure if this is the best approach since I have heard that the residual thyroid follicles may take a few months to regain full function after being suppressed by the over-active thyroid tissue for so long. However, I just want to do what's best (don't we all!)

Thank you so much. I enjoy reading your website and attending your lectures at conferences.

My Response:

First of all, I don't find that free T4 determinations are all that helpful in the diagnosis of feline hypothyroidism (2-4). Many cats treated with radioiodine with maintain low-normal values for both total and free T4 but develop high serum TSH concentrations, a situation commonly referred to as subclinical hypothyroidism in human patients. The problem with our cats, however, is that although most of these cats do remain nonclinical for hypothyroidism, many will develop azotemia that will progressively worsen without treatment with thyroid hormone replacement.

So what I do is as follows: at 30-days post-treatment, I monitor serum concentrations of T4, free T4, and TSH, along with a serum chemistry panel to follow kidney values. If T4 or free T4 values fall into the lower third of the reference range (below 1.5-2.0 µg/dl; reference interval ≈1-4 µg/dl) and TSH rises (above 0.5-0.6 ng/dl; reference range, 0.03-0.03 ng/ml), then the cat is mildly hypothyroid. Some of these cats will recover enough thyroid function to end up as euthyroid, but most remain mildly hypothyroid at both 3 and 6 months, at least based on the finding of high TSH concentrations.

In these cats with mild or subclinical hypothyroidism, I don't like to treat with levothyroxine (LT4) at this time unless evidence of chronic kidney disease (CKD) has developed, with serum creatinine values rising from normal to greater than 2.0 mg/dl. However, this definitely indicates the need for LT4 replacement in order to help maintain renal perfusion and stabilize the serum creatinine concentrations (3-5).

If we decide not to treat (which is generally the case unless new azotemia has developed), then we monitor again with the same thyroid and renal profiles at 3- and 6 months. Again, if T4 falls into the low-normal range (less than 1.5-2.0 µg/dl) and TSH is clearly high (above 0.5-0.6 ng/dl), I would definitely supplement if new or worsening azotemia is detected. If no azotemia is present, I generally continue to monitor and don't supplement with LT4 unless azotemia does develop.

Now, if the serum T4 is below normal and the TSH is clearly high at 3 or 6 months (or later), then the cat has overt hypothyroidism (no longer subclinical) and I would definitely supplement with L-T4 (2-4). Many of these cats are still not very symptomatic, but that may simply be a matter of time. If left untreated for 1 to 2 years, most of those cats will develop classical signs of hypothyroidism (eg, lethargy, hair loss, etc).

So in your case, I would add-in serum TSH to your monitoring protocol. If your owners find that too expensive, then I would replace the free T4 measurement with TSH determination, which is more more helpful in monitoring for cats treated with radioiodine.

References:

1. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. J Vet Intern Med 2010;24:863-869. 
2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: hypothyroidism. Compend Contin Educ Vet 2013;35:E4.  
3. Peterson ME. Diagnosis and management of iatrogenic hypothyroidism In: Little SE, ed. August's Consultations in Feline Internal Medicine: Elsevier, 2014;in press.
4. Peterson ME, Guterl JN.Subclinical iatrogenic hypothyroidism in the cat: Clinical, laboratory, and thyroid scintigraphic findings in 35 cases. J Vet Intern Med 2015;29:448-449.
5. Williams TL, Elliott J, Syme HM. Effect on renal function of restoration of euthyroidism in hyperthyroid cats with iatrogenic hypothyroidism. J Vet Intern Med 2014;28:1251-1255.