Alterations in Metabolism Obesity Hyperthyroid
Module 10 Written Assignment – Exemplars Fill out the Alteration in Metabolism Exemplar Table. Submit your completed assignment by following the directions linked below. Please check the Course Calendar for specific due dates. Alterations in Metabolism Obesity Hyperthyroid Hypothyroid Pathophysiology Risk Factors Assessment Findings (including Labs) Possible Nursing Diagnosis Interventions (including Medications)Alterations in Metabolism Obesity Hyperthyroid
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Thyroid hormones are important for growth, maturation of organ systems, and regulation of metabolism. The thyroid gland manufactures and secretes both thyroxine (T4) and tri-iodothyronine (T3), although the main source of T3 in the body is from conversion of T4 to T3 in peripheral tissues. T3 is more active metabolically than T4. Thyroid hormones circulate both bound to proteins and “free” (ie unbound), with the free fractions being the active fractions.
Thyroid hormone secretion from the thyroid gland is regulated by thyrotropin or thyroid stimulating hormone (TSH) from the anterior pituitary, which in turn is regulated by thyrotropin releasing hormone (TRH) from the median eminence of the hypothalamus.
Thyroid dysfunction in adult horses
Both hypothyroidism and hyperthyroidism have been described in the horse, but true thyroid gland dysfunction is probably much rarer in horses than in some other species, including humans, dogs and cats. A large number of adult horses that are administered thyroid hormones probably have normal thyroid gland function. Abnormalities of thyroid function that have been described in the horse include thyroid gland neoplasia, hyperthyroidism, and hypothyroidism. In addition, certain drugs and a variety of physiologic and/or pathologic conditions can alter serum thyroid hormone concentrations.Alterations in Metabolism Obesity Hyperthyroid
Thyroid gland neoplasia
Thyroid gland neoplasia is not uncommon in horses, particularly in older horses. Histologically, adenomas, carcinomas, adenocarcinomas, and C-cell tumors have been described. Most thyroid gland tumors in the horse are relatively benign, in that they do not tend to metastasize and circulating thyroid hormone concentrations usually remain in the normal range. Many thyroid tumors are found as incidental findings at necropsy examination.
However, some tumors enlarge to the point that they interfere with pharyngeal and/or laryngeal function. These tumors can be removed surgically and the horse managed with thyroid hormone replacement therapy. There are also scattered case reports in the literature of horses that were found to be hypothyroid or hyperthyroid due to a thyroid tumor.
Hyperthyroidism
Hyperthyroidism is extremely rare in horses. There have only been a few cases of hyperthyroidism in adult horses properly documented in the literature, and these have been in association with thyroid hormone producing tumors. Circulating thyroid hormone concentrations are also sometimes temporarily increased in horses exposed to excess iodine, such as in a topical blister. Clinical signs of hyperthyroidism in horses include weight loss, tachycardia, tachypnea, hyperactive behavior, ravenous appetite, and cachexia. Diagnosis is confirmed by measurement of increased circulating concentrations of free fractions of thyroid hormones.
Hypothyroidism
Hypothyroidism is poorly understood in the horse. While hyperthyroidism is rare, the prevalence of true hypothyroidism in adult horses is unknown and its existence is somewhat controversial. Autoimmune thyroiditis, while somewhat common in humans and dogs, has only been described in one report from eastern Europe in the horse, in which histologic lesions compatible with Hashimoto thyroiditis-like disease were found in roughly 20% of 622 horses at a slaughterhouse. Alterations in Metabolism Obesity Hyperthyroid
Although thyroid hormone supplementation is commonly advocated for horses suffering from problems such as laminitis, obesity, recurrent myositis, anhidrosis and poor fertility, proper documentation of hypothyroidism in such cases is often non-existent. Anecdotal reports of beneficial effects of thyroid hormone supplementation in these horses are also largely unsubstantiated.
s a result, individual horses may receive thyroid hormone medication over an extended period of time when they do not really need it. Besides the obvious waste of money, potential health risks associated with unnecessary thyroid hormone supplementation have only minimally been explored in horses. In humans, thyrotoxicosis or over-supplementation with levothyroxine can result in decreased bone density, increased risk of atrial fibrillation, and perhaps increased risk of myocardial infarction or precipitation of congestive heart failure.
Clinical signs of hypothyroidism in adult horses are not well defined. Overweight horses that are easy keepers, with cresty necks, abnormal fat pads, and a predisposition to recurrent laminitis were traditionally described as hypothyroid. However, thyroid function tests in these horses are usually normal. Instead of hypothyroidism, these horses most like suffer from equine metabolic syndrome or pituitary pars intermedia dysfunction. Clinical signs in horses properly documented to be hypothyroid in published case reports include lethargy or work intolerance and alterations in haircoat.
Horses with experimentally-induced hypothyroidism (either through surgical removal of the thyroid glands or by administration of anti-thyroid drugs) demonstrate vague clinical signs. Surgical removal of the thyroid glands of adult horses results in decreased basal heart rate, cardiac output, respiratory rate and rectal temperature, and increased serum concentrations of triglycerides, cholesterol and very low density lipoproteins. However, these changes are mild and do not result in resting values that are clearly outside the normal range for the horse.
Diagnosis of hypothyroidism is based on demonstration of subnormal concentrations of circulating free fractions of serum thyroid hormones, demonstration of increased serum TSH concentration, and/or altered serum thyroid hormone response to TSH or TRH (see section on testing below).
Management of horses that have been properly diagnosed as being hypothyroid, or horses that have undergone thyroidectomy is fairly straightforward. Thyroxine is most commonly used for supplementation and is available in several forms. Iodinated casein contains approximately 1% T4 and is given at 5-15g/horse/day orally. Alterations in Metabolism Obesity Hyperthyroid
The recommended starting dose of levothyroxine is 20 µg/kg horse/day orally. Serum thyroid hormone concentrations should be monitored and dosages of thyroid hormone supplementation adjusted to maintain serum thyroid hormones in the normal range. If a TSH assay becomes commercially available, dosages should be adjusted to normalize TSH.
Physiologic, pathophysiologic and pharmacologic alterations of thyroid function
Certain drugs and various physiologic or pathologic states can alter thyroid hormone synthesis, metabolism or binding, resulting in altered serum concentrations of thyroid hormone. In the horse, these include phenylbutazone or dexamethasone administration, fasting, and strenuous exercise. Diets high in energy, protein, zinc and copper have also resulted in alterations in circulating concentrations of thyroid hormones in horses.
Assessment of thyroid function in adult horses
Because certain drugs and pathophysiologic states can lower serum concentrations of thyroid hormones in otherwise euthyroid horses, it is important that thyroid function tests not be performed while horses are ill, are receiving certain drugs, or are on thyroid hormone supplementation. The author recommends that thyroid hormone testing be performed in horses that have not received any medications for at least 2, and preferably 4 weeks prior to testing. If a horse has been receiving thyroid hormone supplementation without prior documentation of hypothyroidism, the author recommends weaning the horse off supplementation and then testing thyroid function once the horse has not received any supplementation for at least 4 weeks.Alterations in Metabolism Obesity Hyperthyroid
Tests that are currently available for assessment of thyroid function in the horse include measurement of total and free fractions of T4 and T3 and response of these hormones to administration of either TRH or TSH. TRH or TSH stimulation tests are considered to be superior to measurement of baseline thyroid hormone concentrations for evaluation of thyroid function. However, these tests are not routinely performed because of the impracticality of having to take multiple blood samples over time and because TRH and TSH are not readily available commercially or are prohibitively expensive.
If single point-in-time measurement of thyroid hormones is the only option available for evaluation of thyroid status, measurement of free fractions of thyroid hormones (alone or in conjunction with measurement of total amounts of hormone) provides more useful information than does measurement of total amounts of thyroid hormones alone. Measurement of serum TSH concentrations in single samples also will likely aid diagnosis of thyroid status, once a TSH assay for the horse becomes commercially available.
During illness in humans, measurement of serum free T4 by direct methods often underestimates values, when compared to measurements of free T4 after dialysis or ultrafiltration. This also appears to be the case in dogs and horses. Thus, serum concentrations of free T4 measured by equilibrium dialysis are more likely to reflect true thyroid status in ill horses, compared to other methods of free T4 measurement. Measurement of fT4D instead of fT4 may help prevent equine clinicians from mis-diagnosing ill horses as being hypothyroid.
“Normal” values for serum concentrations of thyroid hormones vary somewhat by laboratory due to differences in assay procedures, units of measurement, and populations of horses used to establish the laboratory’s normal values. Therefore, when choosing a diagnostic laboratory, it is important to verify that the laboratory has validated its assays and established reference ranges in a population of normal horses.
Non-thyroidal illness syndrome in the horse
Non-thyroidal illness syndrome has been described in humans, dogs and cats suffering from systemic illnesses. In humans, milder forms of illness result in decreases in serum concentrations of T3, with T4 remaining within the normal range or only slightly decreased. As non-thyroidal illness becomes more severe, total T4 decreases and eventually free T4 also begins to decrease. The magnitude of thyroid hormone suppression has been correlated to severity of disease and to mortality. Alterations in Metabolism Obesity Hyperthyroid
Mechanisms by which thyroid hormones decrease during illness include decreased peripheral conversion of T4 to T3 by 5′-deiodinase, altered binding to serum carrier proteins, and hypothalamic-pituitary dysregulation or suppression. Traditional thought has been that administration of thyroid hormones to patients with non-thyroidal illness syndrome is not beneficial, and might even be detrimental. However, these beliefs have recently been challenged, and the issue remains controversial.
It is thought that non-thyroidal illness syndrome is meant to act as a protective mechanism, with decreased serum concentrations of thyroid hormones resulting in decreased metabolic rate and conservation of lean body mass during times of illness, stress, and decreased food intake. Traditional thought has been that administration of thyroid hormones to patients with non-thyroidal illness syndrome is not beneficial, and might even be detrimental. However, these beliefs have recently been challenged, and the issue remains controversial.
Preliminary data from blood samples collected from horses admitted to the North Carolina State University’s Veterinary Teaching Hospital suffering from a variety of illnesses indicate that, not only are serum concentrations of thyroid hormones decreased, but the magnitude of these decreases are profound in many cases, especially in horses that ultimately died or were euthanized. The contribution of various drugs administered to treat these diseases remains to be determined.Alterations in Metabolism Obesity Hyperthyroid