EVALUATION OF THYROID TO BACKGROUND RATIOS AND COMPARISON OF VARIOUS SCINTIGRAPHIC MEASUREMENTS AND THEIR CORRELATION TO SERUM T4IN HYPERTHYROID CATS

Establishing levels of retained radioactivity in cats receiving radioactive iodine treatment

BACKGROUND

Radioactive iodine (RAI) is considered the gold standard treatment for feline hyperthyroidism. Currently exposure limits to radiation are regulated by national legislation, therefore the length of the isolation period in hospital for cats receiving radioactive treatment varies according to the place where the radioactive facility is located. The aim of this study was to establish when retained radioactivity decreases in cats receiving subcutaneous I-131 to a level that would allow discharge of cats from the hospital while being compliant with current United Kingdom legislation.

METHODS

Clinical records of cats treated with subcutaneous I-131 were retrospectively reviewed. Radioactive emission rates were measured using an external probe. Retained radioactivity below 11 MBq at the point of discharge was required by the initial risk assessment to ensure that a 0.3 mSv dose constraint was maintained for owners following standard cat-owner contact restrictions. Average retained activity for each treatment regimen at the time of discharge was calculated. The biological half-life for iodine retention was also calculated.

RESULTS

Overall, an end activity below 11 MBq was reached at day 11 in 49% of cats, and at day 13 in 91% of cats. These cats were allowed to be discharged according to UK legislation, as long as contact restrictions were applied at home for 2 weeks.

CONCLUSION

Based on our study, an isolation period of 13 days before allowing discharge of cats treated with subcutaneous RAI (I-131) is compliant with current UK legislation.

Relationship between total thyroxine, thyroid palpation and a clinical index in hyperthyroid and healthy cats and cats with other diseases

OBJECTIVES

Present-day diagnosis of hyperthyroidism is often established in the early stages where clinical signs and physical examination findings typically associated with the disease may not yet be present. The purpose of this study was to investigate thyroid palpation score (TPS), total thyroxine (T4), body weight, body condition score and a clinical scoring index, which assesses severity of illness and quality of life, in untreated hyperthyroid cats, healthy cats and cats with non-thyroidal illness.

METHODS

Fifty-five cats with hyperthyroidism, 45 healthy cats and 327 euthyroid cats with non-thyroidal disease were prospectively enrolled. A Kruskal-Wallis test was used to determine any differences between metric data. A χ2 test was applied to compare nominal data between the three subgroups. Correlation between two variables was analysed using Spearman's correlation coefficient.

RESULTS

Almost 80% of the hyperthyroid cats and up to 20% of the healthy cats and cats with non-thyroidal illness had a palpable thyroid gland. Median TPS in hyperthyroid cats was 2, which was significantly higher compared with the other groups. Although there was a significant correlation between TPS and T4 when evaluating all cats, this could not be demonstrated in the three subgroups. Hyperthyroid cats with larger thyroid glands (TPS >3) did not have higher T4 levels. Hyperthyroid cats with lower clinical scores had significantly higher T4 levels compared with hyperthyroid cats with higher scores. Hyperthyroid cats were older and had lower body weights and body condition scores than other cats.

CONCLUSIONS AND RELEVANCE

Hyperthyroid cats were commonly found to have palpable thyroid glands, but they were smaller than previously reported.

Investigation of a novel variable dosing protocol for radioiodine treatment of feline hyperthyroidism

Background

Radioiodine is the treatment of choice for hyperthyroidism in cats. The ideal method of dose determination of radioiodine remains controversial.

Objective

To compare a method of radioiodine dose determination that utilized thyroid scintigraphy with a standard fixed dose for treatment of hyperthyroidism.

Animals

Fifty‐seven and 23 client‐owned hyperthyroid cats in the variable and fixed dose groups, respectively.

Methods

Cats with a percent dose uptake using ^99mTc‐pertechnetate uptake on thyroid scintigraphy <5%, 5%‐10%, and >10% were to receive 3, 3.5, or 4.5 millicuries (mCi) of radioiodine, respectively, administered SC. Radioiodine dose was adjusted according to thyroid gland size as determined by the thyroid:salivary size ratio and categorized as <5:1, 5‐10:1, and >10:1. If the thyroid size fell into a higher dosing category than the percent dose uptake, the dose was increased accordingly. Cats in the fixed dose group received 4.5 mCi. Six months after treatment, cats were determined to be euthyroid, hypothyroid, or hyperthyroid based on serum thyroxine and thyroid stimulating hormone concentrations.

Results

No difference in outcome was found between the variable and fixed dose treatment groups. Euthyroidism, hypothyroidism, and persistent hyperthyroidism developed in 61, 30, and 9% of cats in the fixed dose group compared to 58, 26, and 16%, respectively, in the variable dose group.

Conclusions

A variable dosing method of radioiodine based on percent dose uptake primarily and thyroid gland size secondarily did not improve outcome compared to a standard fixed dose method.