Assessment of treatment outcomes in hyperthyroid cats treated with an orally administered fixed dose of radioiodine

Radioactive iodine dose and survival in cats with hyperthyroidism (2015-2020)

OBJECTIVES

Radioactive iodine (131I) is the preferred treatment for feline hyperthyroidism but neither the optimal 131I dose nor consistent predictors of post-treatment azotaemia have been determined. The aims of the study were to evaluate the relationships between: (1) 131I dose and survival; and (2) pretreatment and post-treatment serum creatinine concentration.

METHODS

Medical records of hyperthyroid cats treated with 131I at a single referral hospital were reviewed. Information regarding signalment, body weight, pretreatment and post-treatment serum total thyroxine concentration (TT4), serum creatinine concentration, 131I dose and survival were determined. Multivariable Cox proportional hazards analysis was used to identify variables associated with survival. Multivariable linear regression analysis was used to identify variables associated with post-treatment serum creatinine concentration.

RESULTS

One hundred and ninety-eight (79 male, 119 female) cats were treated for hyperthyroidism with 131I (median dose 138 MBq; interquartile range 92-168). Median survival time was 1153 days (range 16-1871). Post-treatment serum creatinine (P <0.001) and age (P = 0.049) were significantly associated with survival. Every 10 µmol/l increase in post-treatment serum creatinine concentration and every year increase in age was associated with a 1.07-fold (confidence interval [CI] 1.04-1.11) and 1.17-fold (CI 1.00-1.37) increase in the daily hazard of death, respectively. Pretreatment serum creatinine concentration was directly, and post-treatment serum TT4 concentration was inversely, associated with post-treatment serum creatinine concentration. Every 1 μmol/l increase in pretreatment serum creatinine concentration was associated with an increase in post-treatment serum creatinine concentration of 0.7 μmol/l (SE 0.17; P <0.001). Conversely, every 1 nmol/l decrease in post-treatment serum TT4 concentration was associated with a 1.2 μmol/l (SE 0.61; P <0.001) increase in post-treatment serum creatinine concentration.

CONCLUSIONS AND RELEVANCE

Post-treatment serum TT4 concentration was associated with post-treatment azotaemia, which was associated with survival. Although 131I dose was not directly associated with survival, dosing strategies that minimise post-treatment hypothyroidism and azotaemia could improve patient survival.

The effect of per os vs subcutaneous 123 iodine administration on percentage thyroidal radioactive iodine uptake in normal cats

Background

Historical and recent literature disagree on whether a higher PO ¹³¹I dosage, compared to IV or SC routes, is required for successful resolution of spontaneous hyperthyroidism in cats, necessitating investigation into the effect of PO and injectable radioactive iodine administration on % thyroidal radioactive iodine uptake (RAIU).

Hypothesis/Objectives

To investigate the effect of PO and SC routes of ¹²³I administration on paired % thyroidal RAIU in euthyroid cats. Specifically, a 1.5‐fold difference (50% relative change) was hypothesized, which in absolute terms can be expressed as a 3.25% increase in the mean %RAIU of 7.04% after PO ¹²³I administration to 10.56% after SC dosing.

Animals

Seven healthy euthyroid teaching‐research colony cats.

Methods

A randomized, radiologist‐blinded crossover study comparing %RAIU after PO and SC ¹²³I administration.

Results

Percentage thyroidal RAIU values (mean ± SD; 95% confidence interval) after PO (4.81% ± 1.63%; 3.30%‐6.23%) and SC (5.26% ± 2.43%; 3.01%‐7.51%) ¹²³I administration were associated with a median within‐pair absolute difference of 0.2% (range: min, 0.1%‐max, 4.9%). Statistical significance was not achieved (P  =  .45). Six of 7 cats had a within‐pair absolute difference of 0.1% to 0.9% (relative change of 4%‐20%), but a single outlier cat had a within‐pair absolute difference of 4.9% (relative change of 108%).

Conclusions and Clinical Importance

This study did not detect an effect of ¹²³I administration route on paired % thyroidal RAIU in euthyroid cats. However, a type 2 statistical error due to small sample size is possible.

A dosing algorithm for individualized radioiodine treatment of cats with hyperthyroidism

BACKGROUND

Radioiodine (¹³¹ I) is the treatment of choice for hyperthyroidism in cats, but current ¹³¹ I-dosing protocols can induce iatrogenic hypothyroidism and expose azotemia.

OBJECTIVES

To develop a cat-specific algorithm to calculate the lowest ¹³¹ I dose to resolve hyperthyroidism, while minimizing risk of iatrogenic hypothyroidism and subsequent azotemia.

ANIMALS

One thousand and four hundred hyperthyroid cats treated with ¹³¹ I.

METHODS

Prospective case series (before-and-after study). All cats had serum concentrations of thyroxine (T₄ ), triiodothyronine (T₃ ), and thyroid-stimulating hormone (TSH) measured (off methimazole ≥1 week). Using thyroid scintigraphy, each cat's thyroid volume and percent uptake of ^99m Tc-pertechnatate (TcTU) were determined. An initial ¹³¹ I dose was calculated by averaging dose scores for T₄ /T₃ concentrations, thyroid volume, and TcTU; 80% of that composite dose was administered. Twenty-four hours later, percent ¹³¹ I uptake was measured, and additional ¹³¹ I administered, as needed, to deliver an adequate radiation dose to the thyroid tumor(s). Serum concentrations of T₄ , TSH, and creatinine were determined 6 to 12 months later.

RESULTS

The median calculated ¹³¹ I dose was 1.9 mCi (range, 1.0-10.6 mCi); 1380 cats required additional ¹³¹ I administration on day 2. Of the cats, 1047 (74.8%) became euthyroid, 57 (4.1%) became overtly hypothyroid, 240 (17.1%) became subclinically hypothyroid, and 56 (4%) remained hyperthyroid. More overtly (71.9%) and subclinically (39.6%) hypothyroid cats developed azotemia than euthyroid cats (14.2%; P < .0001).

CONCLUSIONS AND CLINICAL IMPORTANCE

Our algorithm for calculating individual ¹³¹ I doses resulted in cure rates similar to historical treatment rates, despite much lower ¹³¹ I doses. This algorithm appears to lower prevalence of both ¹³¹ I-induced overt hypothyroidism and azotemia.

Treatment failure in hyperthyroid cats after radioiodine (I-131) injection

Background

There is limited published information on the outcome for cats where total thyroxine concentration (TT4) remains elevated after treatment with radioactive iodine (RAI).

Objective

To determine the frequency of, and predictors for, subsequent treatment failure in cats for which TT4 remains elevated at hospital discharge, and to report clinical outcomes for cats requiring repeat treatment.

Animals

One hundred twenty‐one cats with TT4 ≥40 nmol/L after treatment with RAI (out of an original, treated study sample of 959 cats).

Methods

Retrospective study. Data regarding signalment, weight, TT4 concentration (before RAI treatment, at discharge, and percentage change), day of sampling, and I‐131 dose were acquired. Logistic regression was performed to evaluate predictors of treatment failure.

Results

In the 87 cats for which classification was possible, 35 (40%) became euthyroid without further treatment. All TT4 variables and weight normalized RAI dose were independently predictive of subsequent treatment failure. In multivariate analysis, TT4 concentration at discharge (P < .001) and weight normalized RAI dose (P = .04) remained in the final model. All 28 cats with TT4 concentration ≥150 nmol/L at discharge ultimately failed treatment, compared with 13/40 (32.5%) and 11/19 (57.9%) cats with TT4 concentrations of 40‐100 nmol/L and 100‐150 nmol/L, respectively. Of the 52 cats that failed treatment, 14 were subsequently managed medically, 12 underwent thyroidectomy (4 with carcinoma), 14 had repeat RAI treatment which was successful in 12/14 (86%) cats, and 13 had no further treatment.

Conclusions and Clinical Importance

Cats with TT4 >150 nmol/L at discharge after RAI might be candidates for immediate repeat treatment.

Assessment of serum symmetric dimethylarginine and creatinine concentrations in hyperthyroid cats before and after a fixed dose of orally administered radioiodine

Background

Serum symmetric dimethylarginine (SDMA) is a sensitive renal biomarker for detecting early chronic kidney disease (CKD) in nonhyperthyroid cats, but knowledge regarding its performance in hyperthyroid cats remains limited.

Objectives

To determine the relationship between serum SDMA, creatinine and total thyroxine (TT4) concentrations in hyperthyroid cats before (T0) and 3 months after (T1) receiving a PO fixed dose of radioiodine.

Animals

Eighty client‐owned hyperthyroid cats.

Methods

Prospective cohort study. Serum TT4, and SDMA, creatinine concentrations, and urine specific gravity were measured at T0 and T1. Nonparametric tests were used to determine the relationship among SDMA, and creatinine and TT4 concentrations. Agreement between SDMA and creatinine regarding CKD staging at both time points was assessed using Goodman and Kruskal's gamma statistic.

Results

Mean serum SDMA concentration increased after treatment of hyperthyroidism. However, 21 of 75 cats experienced a decrease in SDMA between T0 and T1, whereas creatinine decreased in only 2 cats. A moderate correlation between SDMA and creatinine was seen at T1 (r = 0.53; P < .001) but not at T0 (r = 0.13; P = .25). Where assessable at T1, poor agreement was observed between SDMA and creatinine and CKD stage (Goodman and Kruskal's gamma 0.20; P = .29).

Conclusions and clinical importance

Discordant outcomes between SDMA and creatinine after radioiodine treatment in cats with hyperthyroidism suggest extrarenal factors may interfere with the reliability of SDMA to adequately reflect renal function. As a result, SDMA should not be interpreted in isolation in hyperthyroid cats treated with radioiodine.