Feline endocrinology update

Secondary hypothyroidism following head trauma in a cat

An 18-month-old female neutered domestic short hair cat was examined because of marked polydipsia and stunted growth following head trauma when it was 8 weeks old. Diagnostic evaluation revealed hyposthenuric urine, low concentrations of thyroid hormone and undetectable thyroid stimulating hormone concentrations which did not rise following thyroid releasing hormone administration. Lateral radiographs of the left and right tibiae revealed incomplete mineralisation of the greater tubercle and open physis. An almost empty sella turcica and a greatly reduced pituitary were visible on magnetic resonance images of the brain. A presumptive diagnosis of secondary hypothyroidism and central diabetes insipidus following head trauma was made.

Primary Hyperaldosteronism

Practical relevance Primary hyperaldosteronism is probably the most common adrenocortical disorder in cats. As in humans, it is often unrecognised, which excludes a potentially large number of cats from appropriate treatment.

Patient group Affected cats present at a median age of 13 years (range 5–20 years). A breed or sex predilection has not been documented. The excessive secretion of mineralocorticoids usually leads to hypokalaemia and/or systemic arterial hypertension. Most affected cats present with muscular weakness and/or ocular signs of arterial hypertension.

Diagnostics In any cat presenting with hypokalaemia and/or arterial hypertension, other potential causes should be excluded. The ratio of plasma aldosterone concentration to plasma renin activity (aldosterone:renin ratio) is currently the best screening test for feline primary hyperaldosteronism. Diagnostic imaging is required to differentiate between adrenocortical neoplasia and bilateral hyperplasia, and to detect any distant metastases.

Clinical challenges The differentiation between adrenocortical neoplasia and bilateral hyperplasia is imperative for planning optimal therapy, but the limited sensitivity of diagnostic imaging may occasionally pose a problem. For confirmed unilateral primary hyperaldosteronism, unilateral adrenalectomy is the treatment of choice, and offers an excellent prognosis, but potentially fatal intra- and postoperative haemorrhage is a reported complication and risk factors have yet to be identified.

Evidence base Only a few case reports are available on which to base the optimal diagnostic and therapeutic approach to feline primary hyperaldosteronism. This article reviews the physiology of aldosterone production and the pathophysiology of primary hyperaldosteronism, and summarises the currently available literature on the feline disease. Practical suggestions are given for the diagnostic investigation of cats with suspected primary hyperaldosteronism.

Prevalence of and risk factors for feline hyperthyroidism in Hong Kong

A study was conducted to determine the prevalence of and potential risk factors for feline hyperthyroidism in Hong Kong. Serum total thyroxine (T4) was measured in 305 cats aged 10 years and older that presented at various veterinary clinics in Hong Kong. The prevalence of hyperthyroidism (T4>50 nmol/l) within this population was 3.93% and there was no significant difference in prevalence between healthy (3.16%) and sick (4.37%) cats. Older cats (>15 years) were more likely to be affected and domestic shorthair cats were less likely to be diagnosed with hyperthyroidism than the other breeds combined. No specific association between the development of feline hyperthyroidism and food type was observed. The prevalence of feline hyperthyroidism in Hong Kong was less than that reported for most other parts of the world, despite the presence of previously identified risk factors.

Ocular and systemic manifestations after oral administration of a high dose of enrofloxacin in cats

OBJECTIVE

To characterize the effects of oral administration of a high dose of enrofloxacin to cats.

ANIMALS

24 (12 male and 12 female) young healthy cats.

PROCEDURES

Cats were allocated on the basis of sex into 2 groups (4 males and 4 females/ group) from which 3 subgroups for 3 durations (3, 5, or 7 days) of enrofloxacin (50 mg/kg, PO, q 24 h) or control solution (1 mL of water, PO, q 24 h) administration that began on day -1 were created. Funduscopic examinations were performed daily. Electroretinography (ERG) was performed before and every 2 to 3 days after the start of oral administration. Four cats/study group were euthanized on days 3, 5, and 7, and eyes were collected for light and electron microscopic evaluations.

RESULTS

Neurologic, funduscopic, and ERG abnormalities were evident only in cats administered enrofloxacin. Funduscopic changes (granular appearance or graying of the area centralis) were noticed on or before day 3 (after only 3 days of enrofloxacin administration), with subsequent similar changes along the visual streak. Vascular attenuation (between days 2 and 4) and generalized tapetal hyperreflectivity (between days 5 and 7) followed. Reduction in b-wave ERG amplitude preceded funduscopic changes. Morphologic changes in the photoreceptor layers correlated with duration of enrofloxacin administration, with generalized degenerative changes evident after 3 doses.

CONCLUSIONS AND CLINICAL RELEVANCE

The study indicated that a high dose of enrofloxacin (50 mg/kg/d, PO) induced retinal and systemic changes. Enrofloxacin at 10 times the recommended dosage is acutely toxic to the outer retina of clinically normal cats.

Seroreactivity to A-type retrovirus proteins in a subset of cats with hyperthyroidism

The thyroid gland is afflicted in several endocrine, autoimmune, and malignant diseases. Previous studies detected immunoreactivity against proteins of a human intracisternal A-type retroviral particle type-I (HIAP-I) in serum samples from the majority of patients with Graves' disease, an autoimmune disease of the thyroid that can also affect other organs, most prominently the eyes. To determine whether hyperthyroid animals might provide a model for the retroviral involvement in thyroid autoimmunity, serum samples from 32 cats (21 hyperthyroid and 11 controls) and 10 hypothyroid dogs were examined for immunoreactivity with HIAP-I using a Western blot technique. Of the 21 hyperthyroid cats 15 (71.4%) were HIAP-I positive, while only 2 of 11 (11.8%) control animals without endocrine pathology were positive. No significant correlations were seen between HIAP seroreactivity and serum thyroid hormone levels (T3 and T4), age, gender, treatment history, vaccination status, or weight. No seroreactivity to HIAP-I was detected in hypothyroid dogs. An examination of HIAP-I reactivity in feline leukemia virus (FeLV)-seroconverting cats found that 7/9 (78%) animals viremic for FeLV-A showed an alteration in HIAP serology, whereas only 1/7 (14%) nonviremic animals showed a change in HIAP-I serology. These results suggest that it may be possible to develop an animal (feline) model for the role of retroviruses in thyroid autoimmune diseases.

Feline Acromegaly: A Review of the Syndrome

Acromegaly is characterized by chronic excessive growth hormone (GH) secretion by the pituitary gland. Feline acromegaly is most commonly caused by a functional pituitary tumor. Definitive diagnosis can be difficult because of the gradual disease onset, subtle clinical signs, unavailability of relevant laboratory tests, and client financial investment. The most significant clinical finding of acromegaly is the presence of insulin-resistant diabetes mellitus. Diagnosis is currently based upon brain imaging and measurement of serum GH and/or insulin-like growth factor-1 concentrations. Definitive treatment in cats is not well described, but radiation therapy appears promising.