Iatrogenic hypothyroidism in a hyperthyroid cat treated with

A thirteen-year-old, male, castrated, non-azotemic European Shorthair was presented for treatment of hyperthyroidism. Thyroid scintigraphy using Tc99m showed bilaterally enlarged thyroid glands with an increased thyroid to salivary (T/S) ratio. The cat was treated with an intravenous injection of 4.84 mCi (179MBa) 131I. One year later, the cat showed clinical deterioration, including lethargy, weight loss and a louder heart murmur; iatrogenic hypothyroidism was diagnosed. Concurrently, renal parameters were elevated compared to the pre-treatment values. Supplementation with levothyroxine was started. Four months later, the cat was euthyroid and improved creatinine values were noted. In this case report, the diagnosis and management of iatrogenic hypothyroidism in cats and the interplay with renal function are described. An algorithm with recommendations regarding diagnosis, monitoring and treatment of these cats is presented.


INTRODUCTION
Hyperthyroidism is the most common endocrine disorder in elderly cats (Peterson and Ward, 2007). This disease can be treated pharmacologically, surgically, dietary or with radioiodine therapy (Daminet et al., 2014) (Table 1).
Treatment with 131 I is considered the gold standard in many cats as it is curative and safe. The adminis-

Iatrogenic hypothyroidism in a hyperthyroid cat treated with 131 I
Iatrogene hypothyroïdie bij een hyperthyroïde kat behandeld met 131 I 1 N. De Laet, 1 L. Stammeleer, 2 E. Vandermeulen, 1 D. Paepe, 1 S. Daminet 1 Small Animal Department, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium 2 Department of Medical Imaging of Domestic Animals and Orthopedics of Small Animals, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium nikita.delaet@ugent.be tered radioiodine is taken up by the thyroid glands and the emitted β radiation destroys the nearby abnormal thyroid tissue. The remaining thyroid tissue is mostly atrophied, hence will be spared of most of the radiation (Daminet et al., 2014;Daminet and Hill, 2017). 131 I treatment has a 90-95% success rate (Volckaert et al., 2016). The major disadvantages of 131 I therapy are the need for a licenced, specialized center, the use of radioactive products, the need for hospitalization and the possible development of iatrogenic hypothyroidism (Table 1).
Iatrogenic hypothyroidism has been reported in 3-79% of all 131 I-treated cats, although in most studies, a prevalence of iatrogenic hypothyroidism of approximately 10% has been shown (Meric and Rubin, 1990;Boag et al., 2007;Lucy et al., 2017;. Cats with iatrogenic hypothyroidism rarely show obvious clinical signs. Lethargy, weight gain and anorexia are the most commonly noted. However, these clinical signs are difficult to recognize by the owner since resolution of hyperthyroidism should lead to improvement of the typical clinical signs of hyperthyroidism, including hyperactivity, polyphagia and weight loss. The combination of these vague clinical signs and the possible influence of concurrent illness on total thyroxine (TT4) concentrations (euthyroid sick syndrome) makes diagnosing iatrogenic hypothyroidism after 131 I treatment challenging (Peterson, 2016).
Once iatrogenic hypothyroidism has been diagnosed, the veterinarian can decide to either start treating the cat with thyroid hormone supplementation or continue monitoring. This decision can be difficult to make and depends on a combination of the presence or absence of clinical signs, the time of diagnosis and the renal values of the cat .
Once treatment with levothyroxine has been initiated, frequent re-evaluations should take place to evaluate the clinical response and to measure posttreatment TT4 for therapeutic monitoring. This is important to avoid under-or overdosing of levothyroxine, with persistent hypothyroidism or iatrogenic hyperthyroidism as a consequence. Therapeutic monitoring of total T4 concentrations should take place in every cat treated with levothyroxine, because many factors, including gastrointestinal disease and concurrent administrations of other medications, influence the absorption of levothyroxine (Daminet, 2016).
In this case report, the recommended protocols of diagnosing, treating and re-evaluating iatrogenic hypothyroid cats after radioiodine treatment are described.

CASE DESCRIPTION
A thirteen-year-old, male, castrated European Shorthair was presented at the referring veterinarian with chronic weight loss, polyphagia and being quieter. The diagnosis of hyperthyroidism was confirmed based on increased serum TT4 concentration. Treatment with methimazole was instituted, and the cat was referred to the Small Animal Clinic of the Faculty of Veterinary Medicine, Ghent University, for further work-up and possibly radioiodine treatment. Blood examination (including free T4 (fT4), TT4, hematology and biochemistry), urinalysis, thoracic X-rays, echocardiography and an abdominal ultrasound were performed as part of an extensive work-up for a prospective study. Initial physical examination revealed typical clinical features of hyperthyroidism including muscle atrophy, palpation of a goitre and a systolic heart murmur (Table 2).
Initial blood examination, including biochemistry, hematology and thyroid profile, confirmed the diagnosis of hyperthyroidism with severely increased TT4 and fT4 values and a low normal TSH concentration (Table 3).
Urinalysis prior to the 131 I treatment revealed concentrated urine with mild proteinuria (Table 4). Chest X-rays revealed mild cardiomegaly. Echocardiography visualized mild hypertrophy of the left ventricle with borderline left atrium dilation and dynamic right ventricular outflow tract obstruction (DRVOTO). Abdominal ultrasound did not reveal any significant abnormalities.
Thyroid scintigraphy with Tc-99m showed bilateral enlarged thyroid glands with an increased tracer uptake ( Figure 1). The left thyroid gland was displaced caudally. The thyroid-to-salivary (T/S) ratio was increased in both left (10/1) and right (3.9/1) thyroid glands.
The results obtained from bloodwork and pertechnetate scan in this cat were consistent with the confirmed hyperthyroidism. Treatment with an intravenous injection of 4.84mCi (179MBq) 131 I was instituted. This dose was based on the T/S ratio obtained via scintigraphy, TT4 value and clinical signs (Volckaert et al., 2016). The cat was hospitalized for five days due to radioprotective precautions.
One month post-treatment control included urinalysis, abdominal ultrasound and echocardiography. Significant clinical improvement was noticed ( Table  2). Bloodwork revealed TT4 and fT4 serum concentrations that had dropped below the reference interval and serum TSH concentration well within the upper range of the reference interval. The serum creatinine and urea concentrations were still within reference interval, but had significantly increased compared to the pre-treatment serum concentrations (Table 3). Urinalysis revealed decreased urine concentration compared to the baseline values (Table 4).
The patient returned for a control visit one year after treatment. Blood and urine examinations, scintigraphy, abdominal ultrasound and echocardiography were performed. Significant clinical deterioration including weight loss, mild muscle wastage and a louder heart murmur was noticed and the cat had become significantly more quiet at home ( Table 2). The patient was also hypertensive with a mean blood pressure of 205mmHg (measured with Doppler ultrasonography).
Control bloodwork showed low normal serum TT4 and decreased serum fT4 values, combined with a high serum TSH concentration (Table 3). Concurrent renal disease was also suspected based on an increase in serum values of SDMA, urea and creatinine compared to pre-treatment and significant proteinuria (Tables 3 and 4). The rest of the performed bloodwork, including full hematology, biochemistry and electrolytes, was unremarkable.
Echocardiography showed a mild, focal thickening of the septum with very mild dilation of the left atrium. These findings did not have any clinically relevance at that time point. Abdominal ultrasound didn't show any significant abnormalities.
Control bloodwork was performed four months after the initiation of thyroxine supplementation. The serum TT4 concentration was within normal limits and the urea and creatinine values were mildly decreased. The cat had improved clinically. However, the serum SDMA concentrations were further increased (Table 3).

DISCUSSION
Although radioiodine treatment is the preferred treatment option for hyperthyroidism, in some cats, it also involves disadvantages and complications. One of the main complications is the potential development of iatrogenic hypothyroidism (Meric and Rubin, 1990;Boag et al., 2007;Lucy et al., 2017;. One of the predisposing factors for 131 I therapy induced iatrogenic hypothyroidism is the usage of (too) high doses of radioiodine. The standard dose for treating hyperthyroidism in cats with 131 I used to be approximately 4-5 mCi, but recently, it has been shown that with a dose of 2mCi, most cats with mild-tomoderate hyperthyroidism without an increased frequency of persistent hyperthyroidism three and six  Table 3. Findings on serum biochemistry in a hyperthyroid cat prior to 131 I, and at one, twelve and sixteen months after treatment (T0, T1, T12 and T16, respectively). months post therapy can be successfully treated (Lucy et al., 2017). Although the dosage currently used in the clinic at the Faculty of Veterinary Medicine (Ghent University) has been adapted to 2mCi, the cat in the present case, treated in November 2016, still received 4.84 mCi 131 I, which is nowadays considered to be a high dose. Another predisposing factor for iatrogenic hypothyroidism is the presence of bilateral 131 I uptake on thyroid scintigraphy. These cats, including the cat in the present case report, are twice as likely to develop iatrogenic hypothyroidism compared to cats with only unilateral thyroid pertechnetate uptake (Nykamp et al., 2005). Follow-up of the clinical status and bloodwork after radioiodine treatment is of great importance to be able to make an early diagnosis of iatrogenic hypothyroidism. However, a complete work-up for iatrogenic hypothyroidism in non-azotemic cats is not recommended until three to six months post-treatment, because cats treated with 131 I can become transient hypothyroid. These cats regain a normal thyroid function within three to six months after treatment . Considering that the cat in the present case was hypothyroid one year post-treatment, chances were very low that he would become spontaneously euthyroid again. Azotemic cats that have been treated with 131 I, should be monitored more closely, because treatment for hyperthyroidism, development of iatrogenic hypothyroidism and potential concurrent CKD all decrease the glomerular filtration rate (GFR). Iatrogenic hypothyroidism should be diagnosed and treated as early as possible, since it improves renal function and provides a longer median survival time in these cats Peterson et al., 2017).

Results
The diagnosis of iatrogenic hypothyroidism is currently based on the measurement of serum TT4 and TSH concentrations. Elderly, euthyroid cats often have low TT4 concentrations due to concurrent nonthyroid illnesses (Peterson, 2016). Free T4 concentrations are less influenced by non-thyroidal illness and are therefore considered to be more sensitive for diagnosing iatrogenic hypothyroidism. However, the assays available for the measurement of fT4 concentrations have a variable performance and accuracy. Determining fT4 after equilibrium dialysis (FT4ED) is considered the gold standard. However, FT4ED is an expensive and time-consuming test that is not widely available. In a recent study by Stammeleer et al. (2018), the measurement of fT4 via a chemiluminiscent enzyme immunoassay (FT4CEIA) has shown correlating, but consistently lower results compared to FT4ED. However, approximately 75% of iatrogenic hypothyroid cats have fT4 values within reference interval . Therefore, it is recommended to interpret serum TT4 results in combination with serum TSH concentrations .
Currently, there is no feline TSH assay available. The immulite canine TSH assay (Diagnostic Products Corporation, DPC), a chemiluminescent immunometric assay, is being used to determine TSH concentrations in cats because of the cross-reaction of this assay with feline TSH. Multiple studies have been performed to determine a reference interval (RI) for TSH in healthy, elderly cats. The suggested upper reference limit of feline TSH determined by the cTSH assay varies between 0.15 ng/ml and 0.3 ng/ml (Wakeling, 2010;Williams et al., 2010;Lucy et al., 2017). However, the choice between these two cutoff values will have a major influence on the amount of iatrogenic hypothyroid cats that will be diagnosed after 131 I treatment. This emphasizes the need of further studies to determine the most appropriate cut-off value for this assay in order to be able to diagnose iatrogenic hypothyroidism in cats. In addition, a recent study from Stammeleer et al. (2019) showed that approximately 27% of cats with high cTSH concentrations are euthyroid based on scintigraphy. In that study, TT4 concentrations were within the reference interval in all of these euthyroid cats with increased cTSH concentrations, which shows us the need to interpret TT4 and TSH concentrations together and with caution.
A TSH stimulation test using recombinant human TSH (rhTSH) can also be used to assess the thyroid function. Serum samples should be collected before and 6 hours after IV administration of the exogenous rhTSH. A study by Van Hoek et al. (2010) showed that iatrogenic hypothyroid cats do not show a significant increase in TT4 after the administration of rhTSH, in contrast to healthy cats or cats with non-thyroid illness. One of the main disadvantages of this test is the high cost for a vial of rhTSH. However, once opened, these vials can be stored for four to eight weeks without any loss of biological activity, making it possible to perform multiple TSH stimulation tests with one vial (De Roover et al., 2006). Thyroid scintigraphy is considered the best imaging modality to diagnose iatrogenic hypothyroidism in cats that had received radioiodine treatment. Cats with true iatrogenic hypothyroidism show decreased or absent radionuclide uptake on scintigraphy, whereas cats with low T4 concentrations due to non-thyroidal illness have a normal radionuclide uptake (Peterson, 2013). However, cats that become hypothyroid after methimazole treatment should not be diagnosed with scintigraphy since methimazole can potentially increase the radionuclide uptake, causing a falsely increased T/S ratio (Peterson and Broome, 2015). Unfortunately, thyroid scintigraphy is currently not routinely available for most veterinarians (Peterson, 2013).
Whether or not iatrogenic hypothyroidism should be treated immediately or monitored depends on the renal function of the cat. When bloodwork does not reveal azotemia, the iatrogenic hypothyroidism should not be treated within the first six months after 131 I treatment. However, if the cat is azotemic, treatment with thyroid hormone supplementation has been shown to improve renal function and increase the median survival time in these cases Vaske et al., 2015;Peterson et al, 2017). An algorithm with recommendations regarding diagnosis, monitoring and treatment of hyperthyroid cats treated with 131 I is presented in Figure 3.
Unfortunately, currently, there is no reliable test available that predicts post-treatment azotemia in hyperthyroid cats. Serum creatinine (sCr) concentrations are influenced by body muscle mass, which is usually decreased in hyperthyroid cats. Increased glomerular filtration rate (GFR) in hyperthyroid cats also leads to decreased sCr concentrations, which makes it a less optimal indicator for azotemia in these cats. Direct measurement of GFR is not frequently used in clinical settings because the process is time-demanding and potentially stressful for the cats. Serum symmetric dimethylarginine (SDMA), a byproduct of cellular protein metabolism, is not affected by these extrarenal factors and is therefore suspected to be a better biomarker for the detection of CKD. However, SDMA has a poor sensitivity (33%) to diagnose early kidney disease in hyperthyroid cats (Peterson et al., 2018). In addition, Burresova et al. (2019) showed that cats treated with 131 I show a poor correlation between SDMA and GFR.
One month post treatment, the cat in the present case already showed decreased TT4 concentrations with normal TSH concentrations. Treatment with Lthyroxine was not instituted at this point in time because 1. only a short period had elapsed after treatment, which could reflect transient hypothyroidism; 2. the kidney values remained within the reference interval; 3. no obvious clinical signs of hypothyroi- dism were observed and 4. TSH was not significantly increased.
One year post treatment, the patient was orally administered levothyroxine because of low serum fT4 concentrations, high serum TSH concentration, decreased thyroid visibility on thyroid scintigraphy, increasing kidney values (still within reference interval) and hypertension. At that time point, the cat became lethargic and had mild weight loss (but still a normal body condition score). Lethargy is a common clinical sign in cats with iatrogenic hypothyroidism. Weight loss, however, is unfrequently noticed in hypothyroid cats (Peterson, 2013). Further diagnostic tests were performed in the cat of the present case, including an extensive blood examination, urinalysis, abdominal ultrasound and echocardiography, which didn't reveal other abnormalities besides the mild increase in renal values, proteinuria and hypertension. For this reason, it was recommended to closely follow-up these renal parameters after the initiation of treatment with oral levothyroxine. In addition, the cat showed significant clinical improvement once this treatment was started.
In the present case, the thyroxine and TSH concentrations had not been monitored for six months after treatment, due to a loss of follow-up at that time; otherwise, treatment in this cat might have been initiated sooner.
After starting levothyroxine supplementation, improvement of the mental status and activity level should be noticed within a couple of days. After a couple of weeks, weight loss might be noticed and the hair coat should improve within the first few months. Blood examination should be performed four to eight weeks after the start of the treatment and should include serum urea, creatinine, T4 (TT4 or fT4) and TSH concentrations. Post-treatment blood samples should be collected approximately four hours after the levothyroxine supplementation in order to be able to measure T4 peak serum concentrations. Ideally, T4 concentrations rise and TSH concentrations drop into the reference interval. If these values are not reached, dose adjustment is advised (Daminet, 2016). In the present case, the TT4 concentration increased into the reference interval, renal variables decreased and the cat was clinically stable, hence no dose adjustment was instituted. The serum SDMA concentration in the present case was higher than one year after the 131 I treatment. There are currently no studies available in which the correlation of sSDMA and GFR has been examined in iatrogenic hypothyroid cats treated with levothyroxine, which makes the interpretation of this result challenging.
It is important to note that also medical treatment with methimazole can cause iatrogenic hypothyroidism in cats. Between 20 -40% of cats treated with methimazole become iatrogenic hypothyroid (Aldridge et al., 2015). Many of these cats have TSH concentrations within the reference interval during the first three months of treatment, but iatrogenic hypothyroidism often occurs at a later stage of treatment (Aldridge et al., 2015). Just as in cats treated with 131 I, TT4 or fT4 values should always be interpreted in combination with TSH concentrations. This emphasizes the importance of closely monitoring these cats, including regular bloodwork to check T4 and TSH concentrations in cats treated with methimazole. When low/normal low T4 concentrations and high TSH concentrations are found, the methimazole dose should be lowered, even in the absence of clear clinical signs (Aldridge et al., 2015).

CONCLUSION
Diagnosing, monitoring and treating iatrogenic hypothyroidism in cats can be challenging. Making the decision to start thyroid hormone supplementation can be difficult as iatrogenic hypothyroidism can be transient. Currently, thyroid hormone supplementation is always recommended in azotemic cats as iatrogenic hypothyroidism could negatively influence survival rates in these cats ( Figure 3).