Imaging studies in canine hypothyroidism

Introduction

Acquired primary hypothyroidism is a frequently encountered thyroid disorder in dogs, which could lead to a reduced metabolic rate and impaired function of nearly all body systems. A deficiency occurring at any point within the hypothalamus-pituitary-thyroid axis results in decreased thyroid hormone production (Ko et al., 2018) [11] . The most commonly observed clinical signs in affected dogs include dermatological changes, along with reproductive, neurological, and cardiovascular abnormalities (Guglielmini et al., 2019) [9] . In dogs with thyroid hormone deficiency, common cardiovascular abnormalities include decreased left ventricular systolic function, low QRS voltages, inverted T waves, a weak apex beat, and sinus bradycardia (Beier et al., 2015; Kosková et al., 2022) [2,12] . While thyroid hormones play a role in nervous system function, the precise relationship between hypothyroidism and neurological abnormalities remains unclear. Megaoesophagus is one of the peripheral nervous system disorders that has been documented in dogs with hypothyroidism (Jaggy et al., 1994) [10] . Thyroid hormones regulate numerous metabolic processes in the body, and when their production is disrupted, clinical signs may manifest across multiple organ systems.

Materials and Methods

During the study period from March to November 2023, a total of 5957 dogs were presented to the Veterinary Clinical Complex, NTR College of Veterinary Science, Gannavaram. Of these, 78 dogs exhibiting one or more clinical signs suggestive of hypothyroidism were selected for further evaluation and subjected to thyroid hormone estimation and haematobiochemical analysis. Dogs showing reduced total T4 and free T4 concentrations in conjunction with compatible clinical and biochemical abnormalities were diagnosed as hypothyroid (Bugbee et al., 2023) [4] . Among these, 27 dogs were confirmed to have hypothyroidism. All the confirmed hypothyroid dogs (n = 27) underwent electrocardiographic (ECG) examination, while 15 dogs were further subjected to radiographic, ultrasonographic, and echocardiographic evaluations based on clinical indications.

Electrocardiography

All 27 hypothyroid dogs underwent electrocardiographic (ECG) examination to assess cardiac electrical activity. ECG recordings were obtained in right lateral recumbency using the standard bipolar limb lead system (Lead II) at a paper speed of 25 mm/sec and sensitivity of 10 mm/mV with a BPL CARDIART-6108T machine, following the guidelines described by Tilley and Smith (2008). Parameters such as heart rate, rhythm, P wave, QRS complex, ST segment, and T wave changes were evaluated.

Imaging Studies

Out of 27 hypothyroid dogs, 15 dogs were subjected to thoracic radiography, abdominal radiography, ultrasonography and echocardiography as per clinical indication. Dogs diagnosed with hypothyroidism were subjected to radiographic and ultrasonographic evaluations to assess thoracic and abdominal involvement. Radiography was performed using a 500 mA X-ray machine (Siemen's Medical Equipment Ltd.) with a computerized radiographic system (Kodak Carestream Ltd.). Thoracic radiographs were taken in lateral and ventrodorsal projections, while lateral abdominal radiographs were obtained to evaluate organ size and radiodensity of the liver, spleen, and kidneys following Thrall (2002) [22] . Standard transthoracic echocardiography was performed in 15 dogs using an ALOKA Prosound α6LT (Hitachi Aloka Medical Ltd.) equipped with a 5.5-7.5 MHz phased array transducer. Echocardiographic evaluations included Mmode and two-dimensional imaging, assessment of the LA/AO ratio, and color Doppler studies, following the methodology described by Boon et al. (2011) [3] . Abdominal ultrasonography was carried out at the time of diagnosis using the same equipment with 5-10 MHz curvilinear, convex, or linear transducers, depending on the animal's size (Nyland et al., 2002) [14] . Dogs were positioned in dorsal or lateral recumbency, the abdominal region was clipped and acoustic gel was applied to ensure clear imaging for evaluating visceral organ changes.

Results and Discussion

Electrocardiographic findings, electrocardiographic abnormalities, thoracic radiographic findings, abdominal radiographic findings, abdominal ultrasonographic findings and echocardiographic findings are presented in Tables 1 to 7, respectively. Overall, ECG abnormalities were detected in 15 (55.56%) out of 27 hypothyroid dogs, consistent with the observations of Kumar and Srikala (2013) [13] and Guglielmini et al. (2019) [9] . Panciera (1994) [17] observed that 58 percent of dogs with spontaneous hypothyroidism showed a significant rise in R-wave amplitude (initially <1.0 mV) after Lthyroxine supplementation. Similarly, in the present study, the mean±SE R-wave amplitude (1.06±0.12 mV) was in agreement with the findings of Panciera (1994) [17] and Stephan et al. (2003) [20] . Atrial fibrillation in hypothyroid dogs has also been documented by Gaalova et al. (2008) [6] . These ECG changes might be attributed to the altered inotropic and chronotropic function associated with of thyroxine deficiency (Tappin, 2014) [21] . Factors such as obesity, reduced myocardial mass, and decreased circulating blood volume could contribute to bradycardia and low-voltage R waves, which are often of limited clinical importance unless a pre-existing cardiac condition is present (Panciera, 2001; Scott-Moncrieff, 2007) [16,19] . Thoracic radiographic findings in this study were consistent with those reported by Panciera (2001) [16] , Kumar and Srikala (2013) [13] , and Ojeda et al. (2015) [15] . The megaesophagus observed in three dogs was likely due to generalized polyneuropathy associated with hypothyroidism (Jaggy et al., 1994) [10] . The exact mechanism by which hypothyroidism leads to megaesophagus is unclear. However, it is evident from human studies that, reduced neuronal metabolism may disrupt axonal transport, while mucopolysaccharide accumulation with in Schwann cells and nerve connective tissue may contribute to demyelination (Ko et al., 2018) [11] . Abdominal radiographic and ultrasonographic findings were in line with Gori et al. (2023) [8] , indicating hepatic, gallbladder, and renal involvement in hypothyroid dogs, suggesting a possible metabolic association between hypothyroidism and hepatobiliary alterations. Thyroid hormones play an important role in regulating the function of multiple organs including the kidneys. Thyroid dysfunction alters renal function through both direct renal mechanisms and pre-renal influences, such as metabolic, cardiovascular, and hemodynamic changes. In hypothyroid states, decreased cardiac output, increased peripheral vascular resistance, intrarenal vasoconstriction, reduced renal responsiveness to vasodilators, and lowered activity of the renin-angiotensin-aldosterone system act together to lower renal blood flow (Vargas et al., 2006; Di Paola et al., 2020) [24,5] . Echocardiographic findings (B-mode, M-mode, and Doppler) revealed ventricular enlargement, reduced ejection fraction and fractional shortening, mitral regurgitation, aortic and tricuspid regurgitation, and pericardial effusion which were in partial agreement with Aiswariya (2018) [1] . Mitral valve insufficiency observed in three dogs corroborated Aiswariya's findings, who noted a significant decline in serum T4 levels in dogs with mitral valvular insufficiency (MVI). The myxomatous degeneration of the mitral valve may be attributed to glycosaminoglycan deposition in the valve tissue. Pericardial effusion, noted in three dogs, may have resulted directly from hypothyroidism or could be idiopathic, occurring coincidentally (Ojeda et al., 2015) [15] . The same authors reported that pericardial effusion secondary to hypothyroidism is rare in dogs, describing a cholesterol-rich effusion in a single case. Dilated cardiomyopathy (DCM) recorded in two hypothyroid dogs aligned with the observations of Gomes et al. (2015) [7] and Koskova et al. (2022) [12] , who reported an association between hypothyroidism and DCM. However, Beier et al. (2015) [2] found no association between hypothyroidism and with the development or progression of DCM in dogs. The observed reduction in ejection fraction and fractional shortening, along with increased EPSS and LA/AO ratio, could be attributed to decreased myocardial contractility and low cardiac output (Phillips and Harkin, 2003; Aiswariya, 2018) [18,1] . Nonetheless, Panciera (2001) [16] suggested that although electrical conduction and contractility changes results in detectable echocardiographic abnormalities, their clinical significance is often minimal.

Conclusion

This study demonstrates that multimodal imaging enhances the identification of cardiovascular and systemic alterations in canine hypothyroidism. Combining ECG, radiography, ultrasonography and echocardiography provide more comprehensive and reliable clinical assessment.

Acknowledgement

AcknowledgementsThe authors are thankful to the authorities of NTR College of veterinary science, Gannavaram, Sri Venkateswara Veterinary University for providing necessary facilities for this case study.

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