Thyroid hormones and antithyroid drugs

Cardiovascular manifestations of iatrogenic hyperthyroidism in two dogs

Two dogs were diagnosed with iatrogenic thyrotoxicosis (1 definitive, 1 presumptive). Both showed physical examination findings of agitation, tachypnea, and tachycardia. Sinus tachycardia with supraventricular ectopy was diagnosed in one case, and syncope and atrial flutter was present in the other. Both dogs had concurrent cardiac disease that might have contributed to the severity of their clinical signs. Excessive thyroid hormone supplementation in humans causes supraventricular arrhythmias including sinus tachycardia, supraventricular tachycardia, atrial fibrillation, and atrial flutter. Clinical signs and rhythm abnormalities resolved in both dogs with resolution of the thyrotoxicosis.

Thyroid hormone abnormalities and outcome in dogs with non-thyroidal illness

OBJECTIVES

To document thyroid hormone abnormalities in dogs with non-thyroidal illness and identify markers of prognostic value.

METHODS

Circulating total and free thyroxine, total triiodothyronine and thyrotropin concentrations were measured in 196 dogs with non-thyroidal illness. Clinical signs, previous medications and outcome were recorded in each case. Data were analysed to determine endocrine prognostic factors, and to document the prevalence of thyroid hormone abnormalities.

RESULTS

Total triiodothyronine, and total and free thyroxine concentrations were decreased in 75.9, 34.7 and 4.5 per cent of cases, respectively. Dogs which were euthanased had significantly decreased total triiodothyronine, and total and free thyroxine concentrations compared with those which made a full recovery. Total triiodothyronine concentrations were significantly lower in dogs that were euthanased compared with those which made a partial recovery.

CLINICAL SIGNIFICANCE

Thyroid hormone concentrations may be used as prognostic indicators in dogs with non-thyroidal illness. Low triiodothyronine syndrome may be more common in dogs than previously recognised.

Treatment and therapeutic monitoring of canine hypothyroidism

Thirty-one dogs with spontaneous hypothyroidism were treated with thyroid hormone replacement therapy (THRT) and monitored for approximately three months. Good clinical and laboratory control was ultimately achieved in all cases with a mean L-thyroxine (T4) dose of 0.026 mg/kg administered once daily. There was a significant increase and decrease in circulating total T4 and canine thyroid stimulating hormone (cTSH) concentrations, respectively, after starting THRT. After commencing treatment, 11 cases subsequently required an increase and three cases required a decrease in dose to achieve optimal clinical control. Median (semi interquartile range [SIR]) circulating six-hour post-pill total T4 (53.6 [27.91 nmol/litre) and cTSH (0.03 [0] microg/litre) concentrations were significantly increased and decreased, respectively, in treated dogs that did not require a dose change; corresponding values in treated dogs in which an increase in dose was required were 29.3 (12.7) nmol/litre and 0.15 (0.62) microg/litre, respectively. However, circulating cTSH measurement was of limited value in assessing therapeutic control because, although increased values were associated with inadequate therapy, reference range cTSH values were common in inadequately treated dogs. Lethargy and mental demeanour were typically the first clinical signs to improve, with significant bodyweight reduction occurring within two weeks of commencing THRT. Routine clinicopathological monitoring was of value in confirming a general metabolic response to THRT, but was of limited value in accurately monitoring cases or tailoring therapy in individual cases.

Intravenous administration of levothyroxine for treatment of suspected myxedema coma complicated by severe hypothermia in a dog

A 7-year-old male English Coonhound with suspected myxedema coma complicated by severe hypothermia and metabolic abnormalities was treated with a combination of active external and core rewarming techniques, i.v. and oral administration of levothyroxine, supplemental oxygen, and administration of fluids (0.9% NaCl solution). Myxedema coma develops as a consequence of severe hypothyroidism and is characterized by a hypometabolic, stuporous state. Myxedema coma is associated with a high mortality rate, and most reported cases have involved Doberman Pinschers. Intravenous administration of levothyroxine can be used successfully in combination with oral administration to restore normal metabolic function and assist in warming and thermoregulation, although dosages should be conservative to avoid adverse cardiovascular effects.

Histochemical study of cutaneous mucins in hypothyroid dogs

A dermal mucinosis, visualized as dermal alcianophilic material, is occasionally present in canine hypothyroidism (myxedema). Various histochemical reactions (alcian blue/periodic acid-Schiff [PAS], alcian blue at pH 2.6, alcian blue at pH 1.0, critical electrolytical concentrations with and without dimethylsulfoxide, differential hydrolysis by hyaluronidases) were performed on skin biopsies from six dogs (four females and two males ranging from 8 to 13 years) affected by hypothyroidism, all of them presenting dermal mucinosis in hematoxylin and eosin-stained sections. In these dogs, the only polysaccharidic compound involved in the dermal mucinosis was hyaluronic acid. In this study, hyaluronic acid dermal deposits of hypothyroid dogs were significantly different from those of controls in subepidermal connective tissue and loose reticular connective tissue but not in periadnexal zones. We recommend the combined alcian blue/PAS reaction as a routine technique to assess dermal mucinosis in hypothyroid dogs.

Administration of levothyroxine to euthyroid dogs does not affect echocardiographic and electrocardiographic measurements

The effects of a replacement dose of levothyroxine on electrocardiographic and echocardiographic parameters in euthyroid dogs were examined. Two-dimensional guided M-mode echocardiograms and electrocardiograms were obtained before and after administration of levothyroxine (0.5 mg m-2 twice a day) to 10 euthyroid dogs for a period of eight weeks; four untreated dogs acted as controls. The resting serum total thyroxine concentration was significantly increased in treated dogs during weeks 4 to 8. There were no significant differences in any of the echocardiographic or electrocardiographic measurements resulting from treatment.

Feline hyperthyroidism — A review

Feline hyperthyroidism is the name given to the multisystemic manifestation of excessive concentrations of circulating thyroid hormones, thyroxine (T4) and tri-iodothyronine (T3). The most common cause is a functional thyroid adenoma involving one or both thyroid lobes. Both multinodular adenomatous hyperplasia and multinodular adenomatous goitre are acceptable descriptions for the histopathological findings. Thyroid adenocarcinoma rarely causes hyperthyroidism in the cat, and when present, seldom metastasizes. This is in contrast to the situation in the dog, where thyroid adenocarcinoma is aggressively malignant Grave's disease, an immune-mediated disease, and the most common cause of hyperthyroidism in man, was until recently thought not to occur in the cat. However, a recent study of 29 cats..

Effect of thyrotropin storage on thyroid-stimulating hormone response testing in normal dogs

The stability of reconstituted, refrigerated thyrotropin was evaluated. Thyrotropin (TSH) was reconstituted at the start of the study and stored at 4 degrees C. A TSH stimulation test was performed in eight healthy, euthyroid dogs at weekly intervals for 1 month. In seven of eight dogs, there was no significant difference (P less than 0.05) between the post-TSH T3 concentrations and the post-TSH T4 concentrations for the duration of the study. For one dog, the post-TSH T4 concentration was below the normal post-TSH T4 range following the administration of reconstituted TSH that had been stored 4 weeks. The T3 response to the TSH, however, was normal. This dog responded normally to freshly reconstituted TSH. The results of this study suggest that reconstituted bovine TSH can be stored at 4 degrees C for at least 3 weeks without loss of biologic activity in the dog.