Effect on renal function of restoration of euthyroidism in hyperthyroid cats with iatrogenic hypothyroidism

Urine concentrating ability in cats with hyperthyroidism: Influence of radioiodine treatment, masked azotemia, and iatrogenic hypothyroidism

BACKGROUND

Hyperthyroid cats often have urine specific gravity (USG) values <1.035. It remains unclear how USG changes after treatment, if USG can be used to predict azotemia after treatment, or how iatrogenic hypothyroidism influences USG values.

OBJECTIVES

To determine the proportion of hyperthyroid cats with USG <1.035 vs ≥1.035; if USG changes after treatment; and whether USG <1.035 correlated with unmasking of azotemia or hypothyroidism.

ANIMALS

Six hundred fifty-five hyperthyroid cats treated with radioiodine; 190 clinically normal cats.

METHODS

Prospective, before-and-after study. Hyperthyroid cats had serum thyroxine, thyroid-stimulating hormone, and creatinine concentrations, and USG measured before and 6 months after successful treatment with radioiodine.

RESULTS

Of untreated hyperthyroid cats, USG was ≥1.035 in 346 (52.8%) and <1.035 in 309 (47.2%). After treatment, 279/346 (80.6%) maintained USG ≥1.035, whereas 67/346 (19.4%) became <1.035; 272/309 (88%) maintained USG <1.035, whereas 37/309 (12%) became ≥1.035. Only 22/346 (6.4%) with USG ≥1.035 developed azotemia after treatment, compared with 136/309 (44%) with <1.035 (P < .001). Of cats remaining nonazotemic, 38% had USG <1.035, compared with 20% of normal cats (P < .001). The 137 cats with iatrogenic hypothyroidism had lower USG after treatment than did 508 euthyroid cats (1.024 vs 1.035), but USGs did not change after levothyroxine supplementation. USG <1.035 had high sensitivity (86.1%) but moderate specificity (65.2%) in predicting azotemia after treatment.

CONCLUSIONS AND CLINICAL IMPORTANCE

Hyperthyroidism appears not to affect USG in cats. However, cats with evidence of sub-optimal concentrating ability before radioiodine treatment (USG < 1.035) are more likely to develop azotemia and unmask previously occult chronic kidney disease. Iatrogenic hypothyroidism itself did not appear to affect USG values.

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.

Complex Disease Management: Managing a Cat with Comorbidities

Many older cats often suffer concurrently from multiple conditions. By focusing on the common concerns, rather than conflicting requirements, a management program can be devised. Optimize hydration, nutrition, and ensure comfort though providing analgesia and a low-stress environment in which the patient's feline-specific nature is respected both in the clinic and at home. Additional requirements, such as hyperphosphatemia or hypokalemia, can be met using treatments outside of diet, if necessary.

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

OBJECTIVES

The aims of this study were to describe the treatment outcomes following oral administration of a fixed dose (138 MBq; 3.7 mCi) of radioiodine in hyperthyroid cats and to examine the correlation between total thyroxine (TT4) concentrations before and after treatment.

METHODS

This was a retrospective cohort study that documented the TT4 concentration and clinicopathological parameters at the time of diagnosis and after treatment. Logistic regression was used to assess the relationship between TT4 concentrations before and after treatment. The difference in pre- and post-treatment variables between cats that had TT4 concentrations below or within the reference interval (RI) was compared by the Mann-Whitney U-test.

RESULTS

Of 161 cats, 133 (82.6%) cats had TT4 concentrations within the RI, four (2.5%) cats had TT4 concentrations above the RI and 24 (14.9%) cats had TT4 concentrations below the RI after treatment. The severity of hyperthyroidism at diagnosis, as measured by the percentage of TT4 elevation above the upper limit of the RI, had no impact on the odds of cats having low TT4 concentrations after treatment (odds ratio 1.00; 95% confidence interval 0.96-1.05; P = 0.828).

CONCLUSIONS AND RELEVANCE

When using an orally administered fixed dose of radioiodine for the treatment of feline hyperthyroidism, TT4 concentrations at diagnosis cannot be used to predict TT4 concentrations after treatment. The proportion of cats with TT4 concentrations below the lower limit of the RI after treatment was 14.9%. Further work is required to optimise oral radioiodine dosing to achieve maximal euthyroid outcomes.

Thyroid and renal function in cats following low-dose radioiodine (111Mbq) therapy

OBJECTIVES

To describe the effect of low-dose (111MBq) radioiodine therapy on thyroid and renal function in hyperthyroid cats over a 12-month follow-up period.

MATERIALS AND METHODS

Client-owned hyperthyroid cats underwent low-dose radioiodine therapy and were followed-up for 12 months. Immediately before radioiodine treatment, and at 1, 6 and 12 months afterwards, total thyroxine, thyroid stimulating hormone, serum creatinine and glomerular filtration rate were measured.

RESULTS

Fifteen of the 24 (63%) cats achieved euthyroidism following low-dose radioiodine treatment. The incidence of overt hypothyroidism was six of 24 (25%) cats. Of the six cats developing overt hypothyroidism, three had decreased renal function, with decreased glomerular filtration rate preceding azotaemia in two of these individuals. Transient overt or subclinical hypothyroidism before restoration of euthyroidism was not observed.

CLINICAL SIGNIFICANCE

Low-dose radioiodine is effective treatment for hyperthyroidism in most cats but overt hypothyroidism may develop in some. Concurrent early decline in renal function may only be detected by measuring glomerular filtration rate rather than serum creatinine in some cats. Monitoring following radioiodine treatment should include total thyroxine and thyroid stimulating hormone and measurement of glomerular filtration rate should be considered in non-azotaemic cats.

Changes in thyroid and renal function after bilateral thyroidectomy in cats

BACKGROUND

Iatrogenic hypothyroidism might worsen the prognosis of cats with azotemic CKD after thyroidectomy. Varying thyroxine concentrations influence utility of creatinine in assessing renal function. Symmetric dimethylarginine (SDMA) has limited studies in cats with changing thyroid status.

OBJECTIVES

Thyroid status is stable 6 months post-thyroidectomy. Symmetric dimethylarginine and creatinine are linearly associated without influence from total thyroxine concentration (tT4).

ANIMALS

Electronic records of 2 first opinion practices were searched using the term "thyroidectomy" to include 81 client-owned cats that had undergone bilateral thyroidectomy.

METHODS

Retrospective cross-sectional study assessing thyroid hormone concentrations of 68 cats within 6 months of surgery. A longitudinal study of thyroid status in 23 cats with >18 months follow-up post-thyroidectomy. A generalized estimating equation assessed the associations of bodyweight, tT4 and creatinine concentrations on SDMA concentration.

RESULTS

Sixty-eight cats had follow-up within 6 months. Fifteen cats (22%) had persistent, or recurrent, hyperthyroidism and 33 cats (49%) were hypothyroid. Twenty-three of the euthyroid/hypothyroid cats had long-term follow-up (595-1955 days); 4 cats (17%) remained hypothyroid, 19 cats (83%) were euthyroid (often transiently), and 9 of 23 cats (44%) developed recurrent hyperthyroidism. Symmetric dimethylarginine and creatinine were linearly associated, but hyperthyroid cats had higher SDMA concentrations, relative to creatinine (P = .003).

CONCLUSIONS AND CLINICAL IMPORTANCE

Cats have changes in thyroid function for years after bilateral thyroidectomy, with a high incidence of recurrent hyperthyroidism. Both SDMA and creatinine are affected by thyroxine concentrations, and the effect is greater in hyperthyroid cats.

Assessment of symmetric dimethylarginine as a biomarker of renal function in hyperthyroid cats treated with radioiodine

Background

Measurement of serum creatinine (sCr) and urea nitrogen fail to detect decreased renal function in many hyperthyroid cats because of low muscle mass and glomerular hyperfiltration of affected cats. Serum symmetric dimethylarginine (sSDMA) is an earlier and more sensitive renal biomarker than sCr.

Objective

Evaluate sSDMA as a biomarker of renal function in hyperthyroid cats before (T0) and 1 month after (T1) radioiodine (¹³¹I) treatment.

Animals

Forty‐seven client‐owned hyperthyroid nonazotemic cats were evaluated at T0 and T1.

Methods

A prospective study in which sCr and sSDMA concentrations were determined in 47 hyperthyroid cats at T0 and at T1. Glomerular filtration rate (GFR) was estimated at T0 and T1 in 10 of these 47 cats using plasma exogenous creatinine clearance test.

Results

Serum SDMA was elevated (>14 μg/dL) in 6 of 47 cats at T0 and normalized after treatment in 4 of those cats. All cats remained nonazotemic after treatment. In 10 cats in which GFR was measured, correlation between GFR and sSDMA was low and not significant (τ_b = −0.35, P = .17 at T0 and τ_b = −.22, P = .41 at T1), whereas correlation between GFR and sCr was moderate and significant (τ_b = −0.52, P < .05 at T0 and τ_b = −.53, P = <.05 at T1).

Conclusions and Clinical Importance

Careful interpretation of mildly increased sSDMA with normal sCr in hyperthyroid cats is warranted as sSDMA values might normalize after resolution of hyperthyroidism in some cats. In this population of hyperthyroid cats, sSDMA was poorly correlated with GFR.

Predictive value of scintigraphic (semi-)quantitative thyroid parameters on radioiodine therapy outcome in hyperthyroid cats

Objectives

The outcome of radioiodine therapy in hyperthyroid cats is suspected to be influenced by multiple factors. The degree of activity of the thyroid gland, represented by uptake of sodium pertechnetate or tracer activities of radioiodine by the thyroid gland on thyroid scintigraphy, has been suggested in the literature as one of those. Thyroid gland pertechnetate uptake can be represented by (semi-)quantitative factors such as the thyroid to salivary gland (T/S) ratio, the thyroid to background (T/B) ratio and the percentage technetium uptake by the thyroid glands (%TcU). The aim of this study was to investigate a possible relationship between these thyroid scan parameters and radioiodine therapy outcome.

Methods

Sodium pertechnetate thyroid scans of 75 hyperthyroid cats were retrospectively evaluated and statistical analysis was performed with and without correction for injected radioiodine activity. Three different background regions of interest (ROIs) were used to calculate the T/B ratio and %TcU: ‘neck’, ‘circle’ and ‘copy ROI’.

Results

Higher T/S ratios were found to be significantly related to a persistent hyperthyroid outcome in both analyses. For the T/S ratio, a threshold value of 5.4 was determined, with a sensitivity of 73% and a specificity of 59%. An increased risk for persistent hyperthyroidism compared with a final euthyroid outcome with an increased T/Bcircle ratio was only found to be significant without correction for the activity of radioiodine administered. For the %TcU no statistical significance was reached. Regarding a low total thyroxine outcome, no significant relationships with any of the investigated parameters were found.

Conclusions and relevance

The findings of this study suggest that semi-quantification of thyroid gland uptake is best performed using the T/S ratio. A T/S ratio ⩾5.4 is a possible indicator for an increased risk of persistent hyperthyroidism.

Efficacy of Low-dose (2 millicurie) versus Standard-dose (4 millicurie) Radioiodine Treatment for Cats with Mild-to-Moderate Hyperthyroidism

Background

Radioiodine (¹³¹I) is effective treatment for hyperthyroidism in cats, but optimal dose to restore euthyroidism without inducing hypothyroidism is unclear. Treatment‐induced hypothyroidism can lead to azotemia and reduced duration of survival.

Objective

To compare efficacy and short‐term outcomes of low‐dose ¹³¹I versus higher, standard‐dose ¹³¹I as treatment for hyperthyroidism.

Animals

A total of 189 client‐owned cats undergoing ¹³¹I treatment for mild‐to‐moderate hyperthyroidism (serum T₄ ≥ 4.0 μg/dL and <13.0 μg/dL).

Methods

Prospective, nonrandomized, cohort study comparing treatment with either low‐dose (2 mCi, n = 150) or standard‐dose (4 mCi, n = 39) ¹³¹I. Serum T₄, thyroid‐stimulating hormone (TSH), and creatinine concentrations were measured after 1, 3, and 6 months to determine persistent hyperthyroidism, overt hypothyroidism (low T₄, high TSH), subclinical hypothyroidism (normal T₄, high TSH), and azotemia.

Results

There was no significant difference in prevalence of cats with persistent hyperthyroidism between standard‐ and low‐dose treatment groups at 3 (0% versus 5.3%; P = .34) and 6 (0% versus 3.3%; P = .51) months. Overt (18% versus 1%; P = .0005) or subclinical (46% versus 21%; P = .004) hypothyroidism was more common in cats at 6 months after standard‐dose ¹³¹I. No difference in incidence of azotemia existed between groups, but cats treated with standard‐dose ¹³¹I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001).

Conclusions and Clinical Importance

Low‐dose ¹³¹I is safe and effective for cats with mild‐to‐moderate hyperthyroidism, as evidenced by a cure rate of >95% with reduced frequency of iatrogenic hypothyroidism and azotemia.

2016 AAFP Guidelines for the Management of Feline Hyperthyroidism

Clinical context:

Since 1979 and 1980 when the first reports of clinical feline hyperthyroidism (FHT) appeared in the literature, our understanding of the disease has evolved tremendously. Initially, FHT was a disease that only referral clinicians treated. Now it is a disease that primary clinicians routinely manage. Inclusion of the measurement of total thyroxine concentration in senior wellness panels, as well as in diagnostic work-ups for sick cats, now enables diagnosis of the condition long before the cat becomes the classic scrawny, unkempt, agitated patient with a bulge in its neck. However, earlier recognition of the problem has given rise to several related questions: how to recognize the health significance of the early presentations of the disease; how early to treat the disease; whether to treat FHT when comorbid conditions are present; and how to manage comorbid conditions such as chronic kidney disease and cardiac disease with treatment of FHT. The 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism (hereafter referred to as the Guidelines) will shed light on these questions for the general practitioner and suggest when referral may benefit the cat.

Scope:

The Guidelines explain FHT as a primary disease process with compounding factors, and provide a concise explanation of what we know to be true about the etiology and pathogenesis of the disease. The Guidelines also: Distill the current research literature into simple recommendations for testing sequences that will avoid misdiagnosis and separate an FHT diagnosis into six clinical categories with associated management strategies. Emphasize the importance of treating all hyperthyroid cats, regardless of comorbidities, and outline the currently available treatments for the disease. Explain how to monitor the treated cat to help avoid exacerbating comorbid diseases. Dispel some of the myths surrounding certain aspects of FHT and replace them with an evidence-based narrative that veterinarians and their practice teams can apply to feline patients and communicate to their owners.

Evidence base:

To help ensure better case outcomes, the Guidelines reflect currently available, evidenced-based knowledge. If research is lacking, or if a consensus does not exist, the expert panel of authors has made recommendations based on their extensive, cumulative clinical experience.

ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease

Practical relevance:

Chronic kidney disease (CKD) is one of the most commonly diagnosed diseases in older cats. In most cats, CKD is also a progressive disease and can be accompanied by a wide range of clinical and clinicopathological changes. These ISFM Consensus Guidelines have been developed by an independent panel of clinicians and academics to provide practical advice on the diagnosis and management of this complex disease.

Clinical challenges:

Although CKD is a common clinical problem in cats, the manifestations of disease vary between individuals. Thus there is a need for careful and repeat evaluation of cats with CKD and adjustment of therapy according to individual needs. In addition to addressing problems arising from CKD and improving quality of life (QoL) for the patient, therapy may also target slowing the underlying progression of disease and hence prolonging life. While maintaining QoL is of paramount importance in our patients, this can be challenging when multiple therapies are indicated. In some cases it is necessary to prioritise therapy, given an understanding of what is likely to most benefit the individual patient.

Evidence base:

In preparing these Guidelines, the Panel has carefully reviewed the existing published literature, and has also graded the quality of evidence for different interventions to help to provide practical recommendations on the therapeutic options for feline CKD. This is a field of veterinary medicine that has benefited from some excellent published clinical research and further research findings will undoubtedly modify the recommendations contained in these Guidelines in the future.

Chronic Kidney Disease in Aged Cats: Clinical Features, Morphology, and Proposed Pathogeneses

Chronic kidney disease (CKD) is the most common metabolic disease of domesticated cats, with most affected cats being geriatric (>12 years of age). The prevalence of CKD in cats exceeds that observed in dogs, and the frequency of the diagnosis of CKD in cats has increased in recent decades. Typical histologic features include interstitial inflammation, tubular atrophy, and fibrosis with secondary glomerulosclerosis. In contrast to people and dogs, primary glomerulopathies with marked proteinuria are remarkably rare findings in cats. Although a variety of primary renal diseases have been implicated, the disease is idiopathic in most cats. Tubulointerstitial changes, including fibrosis, are present in the early stages of feline CKD and become more severe in advanced disease. A variety of factors-including aging, ischemia, comorbid conditions, phosphorus overload, and routine vaccinations-have been implicated as factors that could contribute to the initiation of this disease in affected cats. Factors that are related to progression of established CKD, which occurs in some but not all cats, include dietary phosphorus intake, magnitude of proteinuria, and anemia. Renal fibrosis, a common histologic feature of aged feline kidneys, interferes with the normal relationship between peritubular capillaries and renal tubules. Experimentally, renal ischemia results in morphologic changes similar to those observed in spontaneous CKD. Renal hypoxia, perhaps episodic, may play a role in the initiation and progression of this disease.

Evaluation of thyroid-stimulating hormone, total thyroxine, and free thyroxine concentrations in hyperthyroid cats receiving methimazole treatment

Background

Iatrogenic hypothyroidism (IH) after treatment of hyperthyroidism can impair renal function. No study compared the efficacy of measurement of serum free thyroxine by equilibrium dialysis (fT4ed) or thyroid‐stimulating hormone (TSH) concentrations for monitoring cats receiving methimazole.

Objectives

To (1) compare the ability of total T4 and fT4ed concentrations in conjunction with TSH to define thyroid function in hyperthyroid cats receiving methimazole, (2) determine the prevalence of IH in cats receiving methimazole, and (3) examine the relationship between thyroid axis hormones and serum creatinine concentration.

Animals

One hundred and twenty‐five serum samples from hyperthyroid cats receiving methimazole and total T4 concentrations ≤3.9 μg/dL.

Methods

Total T4, fT4ed, and TSH concentrations were measured to evaluate thyroid status and serum creatinine concentration was measured to assess renal function. A low total T4 or fT4ed concentration in combination with an increased TSH concentration defined IH.

Results

Forty‐one cats (33%) had increased TSH concentrations. Of cats with total T4 and fT4ed concentrations below the reference range, 68% and 73%, respectively, had TSH concentrations above the reference range. Only 18% of cats with a normal TSH concentration had an increased serum creatinine concentrations as compared to 39% of those with increased TSH concentrations (P < .001).

Conclusions

Free T4ed does not identify more cats with potential IH as compared to total T4. The IH prevalence was approximately 20%. Measurement of TSH may be more helpful in indicating that azotemia, if present, is at least in part related to IH. Investigation is needed to define TSH assay utility in identifying possible subclinical IH.

Effects of feline hyperthyroidism on kidney function: a review

Chronic kidney disease and hyperthyroidism are two commonly diagnosed conditions in the geriatric feline population, and are often seen concurrently. Management of both diseases is recommended; however, the physiologic implications of both diseases must be understood to ensure the most favorable outcome for each patient. This report reviews the complex interplay between hyperthyroidism and kidney function, as well as the effects of hyperthyroid therapy on kidney function.