Effect of recombinant human thyroid stimulating hormone on serum thyroxin and thyroid scintigraphy in euthyroid cats

Correlation of thyroid hormone measurements with thyroid stimulating hormone stimulation test results in radioiodine-treated cats

Background

Iatrogenic hypothyroidism can develop after radioiodine‐I¹³¹ (RAI) treatment of hyperthyroid cats and can be diagnosed using the thyroid stimulating hormone (TSH) stimulation test.

Objectives

To assess the effect of noncritical illness on TSH stimulation test results in euthyroid and RAI‐treated cats. To assess the correlation of low total‐thyroxine (tT4), low free‐thyroxine (fT4), and high TSH concentrations with TSH stimulation test results.

Animals

Thirty‐three euthyroid adult cats and 118 client‐owned cats previously treated with RAI.

Methods

Total‐thyroxine, fT4, and TSH were measured, and a TSH stimulation test was performed in all cats. Euthyroid control cats were divided into apparently healthy and noncritical illness groups. RAI‐treated cats were divided into RAI‐hypothyroid (after‐stimulation tT4 ≤ 1.5 μg/dL), RAI‐euthyroid (after‐stimulation tT4 ≥ 2.3 μg/dL OR after‐stimulation tT4 1.5‐2.3 μg/dL and before : after tT4 ratio > 1.5), and RAI‐equivocal (after stimulation tT4 1.5‐2.3 μg/dL and tT4 ratio < 1.5) groups.

Results

Noncritical illness did not significantly affect the tT4 following TSH stimulation in euthyroid (P = .38) or RAI‐treated cats (P = .54). There were 21 cats in the RAI‐equivocal group. Twenty‐two (85%) RAI‐hypothyroid cats (n = 26) and 10/71 (14%) of RAI‐euthyroid cats had high TSH (≥0.3 ng/mL). Twenty‐three (88%) RAI‐hypothyroid cats had low fT4 (<0.70 ng/dL). Of the 5 (7%) RAI‐euthyroid cats with low fT4, only one also had high TSH. Only 5/26 (19%) RAI‐hypothyroid cats had tT4 below the laboratory reference interval (<0.78 μg/dL).

Conclusions and Clinical Relevance

The veterinary‐specific chemiluminescent fT4 immunoassay and canine‐specific TSH immunoassay can be used to aid in the diagnosis of iatrogenic hypothyroidism in cats.

Radioactive iodine uptake in hyperthyroid cats after administration of recombinant human thyroid stimulating hormone

BACKGROUND

Radioactive iodine therapy is considered the treatment of choice for hyperthyroidism in cats, but the availability of this modality is limited by costs and hospitalization requirements. Administration of recombinant human thyroid stimulating hormone (rh-TSH) to humans with thyroid neoplasia or nodular goiter can increase thyroidal iodine uptake, thereby allowing the use of lower radioactive iodine doses for treatment. Veterinary studies of this subject are limited, and results are conflicting.

OBJECTIVE

To investigate the effects of rh-TSH administration on thyroidal iodine uptake in hyperthyroid cats.

ANIMALS

Ten client-owned hyperthyroid cats.

METHODS

In this prospective clinical study, cats were administered saline (placebo), 50 μg rh-TSH (low-dose), and 100 μg rh-TSH (high-dose) in randomized crossover design with treatments separated by 7-10 days. After each treatment, thyroid scintigraphy was performed by administering 300 μCi 123 I and assessing radionuclide uptake 8 and 24 hours later. Serum thyroid hormone concentrations were measured at each visit.

RESULTS

Thyroidal percent iodine uptakes (mean ± SD at 8 and 24 hours) in cats treated with placebo (25.2 ± 13.4%, 30.0 ± 12.8%), low-dose (24.1 ± 12.5%, 29.4 ± 13.7%), and high-dose rh-TSH (24.2 ± 16.3%, 30.8 ± 15.3%) were not different (P = .76). Independent of rh-TSH administration, percent iodine uptakes were positively correlated with serum thyroid hormone concentrations.

CONCLUSIONS AND CLINICAL IMPORTANCE

One-time administration of rh-TSH, even at high doses, would not be expected to lower radioactive iodine doses needed for treatment of hyperthyroidism in cats. Investigations of alternate strategies to increase thyroidal uptake of radioactive iodine are warranted.

Recombinant human thyrotropin in veterinary medicine: current use and future perspectives

Recombinant human thyrotropin (rhTSH) was developed after bovine thyrotropin (bTSH) was no longer commercially available. It was approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMEA) as an aid to diagnostic follow-up of differentiated thyroid carcinoma in humans and for thyroid remnant ablation with radioiodine. In addition, rhTSH is used in human medicine to evaluate thyroid reserve capacity and to enhance radioiodine uptake in patients with metastatic thyroid cancer and multinodular goiter. Likewise, rhTSH has been used in veterinary medicine over the last decade. The most important veterinary use of rhTSH is thyroidal functional reserve testing for the diagnosis of canine hypothyroidism. Recent pilot studies performed at Ghent University in Belgium have investigated the use of rhTSH to optimize radioiodine treatment of canine thyroid carcinoma and feline hyperthyroidism. Radioiodine treatment optimization may allow a decreased therapeutic dosage of radioiodine and thus may improve radioprotection. This review outlines the current uses of rhTSH in human and veterinary medicine, emphasizing research performed in dogs and cats, as well as potential future applications.

Thyroid stimulation with recombinant human thyrotropin in healthy cats, cats with non-thyroidal illness and in cats with low serum thyroxin and azotaemia after treatment of hyperthyroidism

This study investigated the recombinant human thyrotropin (rhTSH) stimulation test in healthy cats (group 1), cats with non-thyroidal illness (group 2) and cats with low serum total T(4) (TT(4)) and azotaemia after (131)I treatment (group 3). Serum TT(4) responses and thyroidal pertechnetate uptake after administration of 25 microg rhTSH IV were assessed. Baseline serum TT(4) was significantly lower in group 3 compared with group 1, but not between other group pairs. Serum TT(4) increased significantly in groups 1 and 2 but not in group 3 after rhTSH administration. Post-rhTSH serum TT(4) concentrations differed significantly between groups 1 and 3 and groups 2 and 3, but not between groups 1 and 2. Thyroid/salivary gland uptake ratio (T/S uptake ratio) differed only significantly between groups 1 and 3. Stimulation with rhTSH is valuable to differentiate euthyroidism from iatrogenic hypothyroidism in cats.