Altered expression of G proteins in thyroid gland adenomas obtained from hyperthyroid cats

Feline Comorbidities: Balancing hyperthyroidism and concurrent chronic kidney disease

PRACTICAL RELEVANCE

Both hyperthyroidism and chronic kidney disease (CKD) are common long-term conditions in older cats, which might be diagnosed concurrently or develop at different times. Hyperthyroidism may mask the presence of CKD, and vice versa, by various mechanisms that are described in this review. Hyperthyroidism treatment options should be carefully considered when CKD has also been diagnosed.

CLINICAL CHALLENGES

Although it can be difficult to diagnose hyperthyroidism and CKD simultaneously, given that one condition may mask the other, it is important to consider the presence of both diseases when examining an older cat presenting with vomiting, weight loss, polyuria/ polydipsia, anorexia or sarcopenia. The concurrent presence of hyperthyroidism and CKD requires careful monitoring of glomerular filtration rate biomarkers, and adequate and prompt support of kidney function when normal thyroid function is re-established. Iatrogenic hypothyroidism is a recognised complication of all of the treatment options for hyperthyroidism, and increases the risk of azotaemia. Therapy with levothyroxine is recommended for cats that are hypothyroid and azotaemic.

EVIDENCE BASE

The information in this review draws on current literature and guidelines related to the pathophysiology, diagnosis and treatment recommendations for feline hyperthyroidism and CKD.

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.

A critical review of food-associated factors proposed in the etiology of feline hyperthyroidism

Since the first description of feline hyperthyroidism (HT) in 1979, several studies have been undertaken to define the etiology of the disease. Epidemiologic studies, after investigating non-food- and food-associated factors, suggest a multifactorial etiology. However, in the absence of prospective cohort studies that can confirm a cause-and-effect relationship between HT and associated risk factors, no causative factor for HT has been identified to date. Feline HT resembles toxic nodular goiter in humans, with autonomously functioning upregulated iodide uptake systems. Contribution of the diet to HT development remains controversial. The purpose of this paper is to review critically the reported food-associated risk factors for HT.

Animal models of disease: feline hyperthyroidism: an animal model for toxic nodular goiter

Since first discovered just 35 years ago, the incidence of spontaneous feline hyperthyroidism has increased dramatically to the extent that it is now one of the most common disorders seen in middle-aged to senior domestic cats. Hyperthyroid cat goiters contain single or multiple autonomously (i.e. TSH-independent) functioning and growing thyroid nodules. Thus, hyperthyroidism in cats is clinically and histologically similar to toxic nodular goiter in humans. The disease in cats is mechanistically different from Graves' disease, because neither the hyperfunction nor growth of these nodules depends on extrathyroidal circulating stimulators. The basic lesion appears to be an excessive intrinsic growth capacity of some thyroid cells, but iodine deficiency, other nutritional goitrogens, or environmental disruptors may play a role in the disease pathogenesis. Clinical features of feline toxic nodular goiter include one or more palpable thyroid nodules, together with signs of hyperthyroidism (e.g. weight loss despite an increased appetite). Diagnosis of feline hyperthyroidism is confirmed by finding the increased serum concentrations of thyroxine and triiodothyronine, undetectable serum TSH concentrations, or increased thyroid uptake of radioiodine. Thyroid scintigraphy demonstrates a heterogeneous pattern of increased radionuclide uptake, most commonly into both thyroid lobes. Treatment options for toxic nodular goiter in cats are similar to that used in humans and include surgical thyroidectomy, radioiodine, and antithyroid drugs. Most authorities agree that ablative therapy with radioiodine is the treatment of choice for most cats with toxic nodular goiter, because the animals are older, and the disease will never go into remission.

Worldwide prevalence and risk factors for feline hyperthyroidism: A review

Since first reported in the late 1970s, there has been a steady but dramatic increase in the worldwide prevalence of hyperthyroidism in cats. It is now regarded as the most common feline endocrine disorder, with diabetes mellitus coming a close second. Not only is there evidence for an increased worldwide prevalence of feline hyperthyroidism, but also for geographical variation in the prevalence of the disease. Despite its frequency, the underlying cause(s) of this common disease is or are not known, and therefore prevention of the disease is not possible. Due to the multiple risk factors that have been described for feline hyperthyroidism, however, it is likely that more than one factor is involved in its pathogenesis. Continuous, lifelong exposure to environmental thyroid-disruptor chemicals or goitrogens in food or water, acting together or in an additive fashion, may lead to euthyroid goitre and ultimately to autonomous adenomatous hyperplasia, thyroid adenoma and hyperthyroidism. This review aims to summarise the available published evidence for the changes observed in the worldwide prevalence of the disease, as well as risk factors that may contribute to development of hyperthyroidism in susceptible cats.

Evaluation of activation of G proteins in response to thyroid stimulating hormone in thyroid gland cells from euthyroid and hyperthyroid cats

OBJECTIVE

To evaluate alterations in ligand-stimulated activity of G proteins in thyroid gland cells of hyperthyroid cats.

SAMPLE POPULATION

Membranes of thyroid gland cells isolated from 5 hyperthyroid cats and 3 age-matched euthyroid (control) cats immediately after the cats were euthanatized.

PROCEDURES

Isolated thyroid cell membranes were treated with thyroid-stimulating hormone (TSH), and activation of G protein was quantified by measurement of the binding of guanosine triphosphate gamma labeled with sulfur 35 (GTPgamma(35)S). The separate effects of G-protein inhibitory (G(i)) and G-protein stimulatory (G(s)) proteins were determined by the use of pertussis toxin and cholera toxin, respectively.

RESULTS

Thyroid cell membranes from hyperthyroid cats had higher basal GTPgamma(35)S binding than did thyroid cell membranes from euthyroid cats. Thyroid cell membranes from hyperthyroid and euthyroid cats had a concentration-dependent increase in TSH-stimulated GTPgamma(35)S binding over the TSH range of 0 to 100 mU/mL, with maximal activity at 1 to 100 mU/mL for both. The percentage increase in GTPgamma(35)S binding stimulated by TSH was similar in magnitude between the membranes from hyperthyroid and euthyroid cats. The TSH-stimulated activation of G(s) and G(i) was not different between euthyroid and hyperthyroid cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Ligand-stimulated activation of G proteins was the same in thyroid cell membranes obtained from hyperthyroid and euthyroid cats. Therefore, alterations in inherent G(s) or G(i) activities did not appear to be part of the pathogenesis of hyperthyroidism in cats.

Urinary iodide concentration in hyperthyroid cats

OBJECTIVE

To compare concentrations of urinary iodide (UI) in euthyroid and untreated hyperthyroid cats.

ANIMALS

118 euthyroid and 88 hyperthyroid client-owned cats from 2 nonreferral veterinary practices.

PROCEDURES

Iodide concentration was measured in 5 urine samples collected every 3 to 12 months from selected cats, and variability of results between euthyroid cats and hyperthyroid cats prior to the diagnosis of hyperthyroidism was evaluated via 1-way ANOVA, after logarithmic transformation of UI concentrations (logUIs). The UI concentration in hyperthyroid cats was measured at diagnosis and 2 to 6 weeks and 3 to 6 months after treatment for hyperthyroidism. The pretreatment logUI in hyperthyroid cats was compared with that in euthyroid cats, taking into account the effects of renal function on UI concentration. Iodine intake was estimated in euthyroid cats following calculation of the volume of daily urine output, with a fixed value for iodine concentration in feces.

RESULTS

The variability of UI concentrations did not differ significantly between hyperthyroid (n = 10) and euthyroid (8) cats. The logUI increased 2 to 6 weeks after initiation of treatment in hyperthyroid cats (n = 80) and was lower in azotemic versus nonazotemic cats. Hyperthyroid cats had a lower logUI than euthyroid cats, and there was no evidence of deficient iodine intake in euthyroid cats.

CONCLUSIONS AND CLINICAL RELEVANCE

The logUI was lower in cats with azotemia and with untreated hyperthyroidism, compared with that in euthyroid cats from the same population. Additional studies are needed to determine whether iodine intake plays a role in the development of hyperthyroidism in cats.

Subclinical hyperthyroidism in cats: a spontaneous model of subclinical toxic nodular goiter in humans?

INTRODUCTION AND OBJECTIVES

Hyperthyroidism in cats, caused by nodular hyperplasia or adenomas, is clinically and histologically similar to toxic nodular goiter in humans. Subclinical hyperthyroidism in humans is defined as low thyrotropin (TSH) in conjunction with within-reference-range thyroid hormone concentrations, but has not previously been defined in cats. The objective of this study was to test the hypothesis that euthyroid senior cats with low TSH have histological evidence of thyroid nodular hyperplasia and/or adenoma.

DESIGN

Thyroid glands removed postmortem from four groups of cats (n = 73) were examined histologically and scored in a blinded fashion. Clinically euthyroid senior (>7 years) cats were divided into two groups dependent on their TSH concentration--TSH below the limit of quantification (LOQ) of the assay (<0.03 ng/mL; n = 15; UndetectableTSH group) and TSH above the LOQ (>or=0.03 ng/mL; n = 31; DetectableTSH group)--using archived plasma samples, collected 0-6 months antemortem. Thyroids were also scored for two control groups: Young group (cats <6 years old; n = 13) and Hyperthyroid group (clinically and biochemically hyperthyroid cats; n = 14).

MAIN OUTCOME

Cats in the UndetectableTSH group had a higher frequency of nodules, a greater percentage of abnormal thyroid tissue, and a higher overall histopathological grade than cats with detectable TSH had.

CONCLUSION

Euthyroid (as defined by total thyroxine) senior cats with low TSH are likely to have histological evidence of nodular thyroid disease, and such cats could be considered to be subclinically hyperthyroid.

Etiopathologic Findings of Hyperthyroidism in Cats

None of the studies to date have isolated a single dominant factor that could be incriminated in the development of hyperthyroidism in cats. Rather, most of the studies provide further evidence of the widely held view that hyperthyroidism is a multifactorial disease in this species. At this time, the most likely candidates include one or more of the goitrogenic chemicals that have been shown to be present in cat food or the cat's environment. In addition, mutations of the thyroid stimulating hormone receptor gene or mutations of its associated G proteins seem to play an important role in the pathogenesis of this disease.

Expression of inhibitory G proteins in adenomatous thyroid glands obtained from hyperthyroid cats

OBJECTIVE

To identify within guanosine triphosphate-binding proteins (G proteins) the subset of inhibitory G proteins (G) that have decreased expression in adenomatous thyroid glands obtained from hyperthyroid cats.

SAMPLE POPULATION

Adenomatous thyroid glands obtained from 5 hyperthyroid cats and normal thyroid glands obtained from 3 age-matched euthyroid cats.

PROCEDURE

Expression of G(i1), G(i2), and G(i3) in enriched membrane preparations from thyroid glands was quantified by use of immunoblotting with G(i) subtype-specific antibodies.

RESULTS

Expression of G(i2) was significantly decreased in tissues of hyperthyroid glands, compared with expression in normal thyroid tissue. Expression of G(i1) and G(i3) was not significantly different between normal thyroid tissues and tissues from hyperthyroid glands.

CONCLUSIONS AND CLINICAL RELEVANCE

A decrease in G(i2) expression decreases inhibition of adenylyl cyclase and allows a relative increase in stimulatory G protein expression. This results in increased amounts of cAMP and subsequent unregulated mitogenesis and hormone production in hyperthyroid cells. Decreased G(i2) expression may explain excessive growth and function of the thyroid gland in cats with hyperthyroidism.

Thyrotropin-stimulated DNA synthesis and thyroglobulin expression in normal and hyperthyroid feline thyrocytes in monolayer culture

Feline hyperthyroidism is a common, spontaneous disease in older cats that is similar clinically and histopathologically to human toxic multinodular goiter (TNG). In this study, the functional response of feline normal thyroid (NT) and hyperthyroid (HT) cells grown in monolayer culture to thyrotropin (TSH) was determined. Basal levels of DNA synthesis were similar in NT and HT cells. TSH stimulated concentration-dependent DNA synthesis in NT and HT cells, with maximal stimulation seen at 1 and 10 mU/mL TSH in NT and HT cells, respectively. HT cells had higher basal levels of thyroglobulin (Tg) expression. TSH stimulated Tg expression in NT and HT cells in a concentration-dependent fashion, with maximal activity at 0.5 and 5 mU/mL TSH, respectively. These results demonstrate that NT and HT cells in monolayer culture exhibit growth and functional responses to TSH. HT cells have higher basal Tg expression than NT cells and require higher TSH concentrations to stimulate DNA synthesis and Tg expression, two measures of thyroid cell activation. These data support the idea that feline hyperthyroidism is caused by cell abnormalities, resulting in dysregulated growth and hormone synthesis, and emphasize its importance as an animal model for TNG.

Relationship between semi-quantitative thyroid palpation and total thyroxine concentration in cats with and without hyperthyroidism

In 155 cats, both with and without clinical signs of hyperthyroidism, total thyroxine (TT4) concentrations were compared to a sensitive, semi-quantitative thyroid palpation technique. On the basis of TT4 concentrations, 23 of the 155 cats were classified as hyperthyroid. The size of individual thyroid glands was scored between '0' (non-palpable) and a maximum of '6'. One or more enlarged thyroid glands (score >0) were palpated in 22 of the 23 hyperthyroid cats and in 78 of the 132 euthyroid cats. However, none of the 132 euthyroid cats had a thyroid lobe score of greater than '3' whereas 18 of the 23 hyperthyroid cats had a thyroid lobe score of '4' or greater, and in two of the five that had scores below '4' there was evidence of intrathoracic functional thyroid tissue on scintigraphy.

Feline thyroid adenomas are in part associated with mutations in the G(s alpha) gene and not with polymorphisms found in the thyrotropin receptor

The etiopathogenesis of feline thyrotoxicosis is unknown. The transmembrane part (gene codons 480-640) of the thyrotropin receptor (TSHR) gene of hyperthyroid cats has already been investigated for the presence of stimulating mutations. No mutations were found in this part of the TSHR gene. We have investigated the TSHR gene codons 66-530 for gene mutations in 10 hyperthyroid cats and in 1 euthyroid cat. This part of the TSHR gene encodes the transmembrane part as well as most of the extracellular part of the receptor. The G(s alpha) gene of these cats was also sequenced and subjected to mutational analysis. Although our study revealed a polymorphism in the TSHR gene, no association was found with tumor formation. In 4 of 10 cats with hyperthyroidism a G(s alpha) gene mutation was found. This work suggests that mutations in the extracellular or transmembrane part of the TSHR gene are not likely the cause of feline hyperthyroidism. Mutations in the G(s alpha) gene, however, may play a role in the etiopathogenesis of this disease.

Feline endocrinology update

This article highlights the advances in feline endocrinology, excluding diabetes mellitus and hyperadrenocorticism, which have recently been reviewed elsewhere. The goal will be to provide clinically relevant information regarding pathogenesis, diagnosis, and treatment options for these feline endocrine disorders.