Hypothyroidism part two diagnosis, treatment and nursing

Transcription

1 Vet Times The website for the veterinary profession Hypothyroidism part two diagnosis, treatment and nursing Author : Gemma Reid Categories : RVNs Date : July 1, 2008 Gemma Reid RVN, looks at issues surrounding the diagnosis of this condition, treatment options and the role of the nurse THE first part of this article covered normal thyroid gland function, the causes of hypothyroidism and clinical signs. This final part deals with the difficulties of diagnosis, treatment, and the nursing care involved. Diagnosis This is where things become complicated. Because hypothyroidism affects so many systems, the clinical signs overlap with many other conditions. There is no laboratory test that will 100 per cent diagnose hypothyroidism either, so it becomes a process of elimination and problem solving for the veterinary surgeon. The first step, therefore, is to rule out other illnesses that may be more clinically apparent for example, hyperadrenocorticism (Cushing s disease) and that may also cause low thyroid hormone levels. There is a collective name for these other conditions, which is sick euthyroid syndrome, meaning the animal has altered levels of thyroid hormones, but as far as the thyroid gland is concerned, it is euthyroid that is, has normal thyroid gland function. Certain drugs can also interfere with test results, particularly steroids, potentiated sulphonamides (eg trimethoprim-sulfadiazine) and barbiturate anticonvulsants (eg phenobarbitone). Ideally, all drug therapy should be withdrawn four to six weeks before assessing thyroid function. 1 / 12

2 Endocrine tests TT4 (total T4). This is a good basic screening test. A hypothyroid dog is very unlikely to have a normal TT4 level (see ), but a low level of TT4 could also be sick euthyroid syndrome, or affected by concurrent therapy. Sight hounds are known to routinely have low TT4 levels, and, confusingly, levels in all dogs can fluctuate throughout the day and become subnormal at random times1. Endogenous thyrotropin (TSH). In theory, TSH levels are increased in primary hypothyroidism, as the decreased circulating thyroid hormone concentrations result in a lack of negative feedback to the HPT axis and cause the pituitary to increase its TSH production, trying to get the thyroid gland to produce more T4. An elevated TSH level may also be seen in dogs receiving sulphonamides, or those recovering from nonthyroidal illnesses, and, to further complicate the issue, per cent of dogs that are hypothyroid have normal TSH levels. However, it remains a useful test in conjunction with measuring TT4, as a raised TSH (>0.6ng/ml) together with decreased TT4 would be highly suggestive of hypothyroidism1, 2. A test for TSH in cats is not currently available. ft4 (free T4). Measurement of this biologically active component of T4 is less affected by nonthyroidal illness and drug therapy (although barbiturate anticonvulsants may lower levels). For dependable results it needs to be measured by equilibrium dialysis (ED), which is an expensive test compared to TT4, but decreased values would be more specific for hypothyroidism1, 2. TT3 (total T3). This is not recommended for assessing thyroid function as the majority of T3 is produced by local deiodination of T41. Thyroglobulin antibodies (TgAb). This test is generally not recommended either, as the results can be misunderstood or misinterpreted. TgAbs are produced during the development of lymphocytic thyroiditis, and while their presence can indicate thyroid pathology, not all hypothyroid dogs have lymphocytic thyroiditis so a negative result does not rule out hypothyroidism1, 2. Also, euthyroid dogs can demonstrate TgAbs. TSH stimulation test. Stimulating the thyroid gland by administering exogenous bovine TSH (btsh) and measuring the response used to be the gold standard test for hypothyroidism. Unfortunately, btsh is no longer widely available; a chemical-grade btsh has caused adverse reactions and even death, and further studies need to be carried out on recombinant human TSH, so it is not a widely used test at the moment1, 2, 3. TRH stimulation test. This test can be used to exclude hypothyroidism from the list of differentials, as a hypothyroid dog would not be able to respond normally to exogenous TRH administration. Unfortunately, many dogs react adversely to the IV administration of TRH, and because the results are no more conclusive than the more basic tests, it is not usually recommended1, 2. 2 / 12

3 It is important to remember that the tests detailed above must be interpreted in conjunction with the clinical signs that the animal is showing, in order to reach the correct diagnosis. Measuring response to thyroid supplementation (therapeutic trials) Starting the patient on medical treatment for hypothyroidism as a means of diagnosis is controversial. Many euthyroid dogs will also respond positively to thyroid supplementation (eg show increased hair growth). There is a risk of iatrogenic hyperthyroidism with excessive supplementation (although it s a low risk as physiologic regulation of T4 to the more active T3 is preserved4), and the medication suppresses TSH and, therefore, thyroid function, so can confuse later assessment of the thyroid. It is generally agreed to begin trial therapy only when other diagnostic tests are exhausted, to measure the response carefully, and when there is a good clinical response to temporarily withdraw treatment and see if signs recur1, 2. Other tests Thyroid biopsy. This is invasive, not always diagnostic, and identification at surgery of an atrophied thyroid gland can be difficult, so it s rarely recommended5. ECG. Hypothyroid dogs may show a slow heart rate, weak apex beat, low QRS voltages and inverted t-waves. Ophthalmologic examination. Reported changes include corneal lipidosis, ulceration, uveitis and KCS among others4. Neurology exam. To assess any neurological dysfunction as mentioned previously. Mild, non-regenerative anaemia. This is seen in per cent of cases, due to decreased iron absorption, decreased EPO levels and response to EPO in bone marrow. Fasting hypercholesterolaemia. A high cholesterol level, > 13 mmol/l in a fasted dog, is seen in 75 per cent of hypothyroid cases, and can cause some of the ocular changes associated with hypothyroidism. However, it is not pathognomonic (ie, not exclusive) for hypothyroidism and may occur with other diseases, such as Cushing s and diabetes mellitus. Treatment The generic name of the drug used to treat hypothyroidism is L-thyroxine sodium, which is available in several forms for example, as Soloxine (manufactured by Virbac) and Forthyron (manufactured by Ceva Animal Health). The starting dose for dogs is 0.02 mg/kg given orally every 3 / 12

4 12 hours, and the response is evaluated by measuring TT4 levels four to six weeks later, the blood sample being taken four to six hours after morning pill administration. The aim is for peak TT4 levels to be between nmol/l (at the high end of reference range or slightly above), and during the first six to eight months of treatment these levels will need to be checked every six to eight weeks as the metabolism changes and normalises. Eventually a dose of 0.02 mg/kg every 24 hours should be adequate, with TT4 levels being measured only once or twice a year4. The starting dose for hypothyroid cats is 0.05 to 0.1 mg/cat given orally every 24 hours, with the same monitoring and subsequent dose adjustments as for dogs. Response to treatment and prognosis Response to treatment is normally very positive. Metabolic signs often start to improve within days, with dermatological, weight and clinicopathological improvements taking longer, but evident usually at six weeks. Overall, within three months all signs of hypothyroidism should be resolved, making it a very satisfying disease to treat! The long-term prognosis is also excellent, and quality of life should be very good, provided the dog is monitored regularly and the treatments are given correctly at home. The nurse s role The main role of the nurse, as described in Veterinary Nursing third edition (Lane and Cooper, p 469) is to: 1. monitor vital signs; 2. assist the veterinary surgeon with diagnostic tests; 3. administer medications; 4. feed an appropriate diet; and 5. monitor clinical signs. As hypothyroidism is a slow, progressive disease, it is unlikely to present as an emergency (with the exception of myxoedema coma, see ). So it may appear at first that the nurse s tasks are not particularly challenging, but I think having a greater understanding of this disease and the difficulties faced in diagnosing it will make the VN s role more satisfying. An awareness of how the vital signs may change, how important it is to give the medication at the correct dose and time, and to look through and discuss the test results with the vet should hopefully increase your interest and involvement with these cases. 4 / 12

5 References 1. Wray, J Canine hypothyroidism. Personal communication. 2. Dixon, R (2001). Recent developments in the diagnosis of canine hypothyroidism, In Practice Ramsey, I (1997). Diagnosing canine hypothyroidism, In Practice Scott-Moncrieff, J C and Guptill-Yoran, L (2000). Hypothyroidism. In: Textbook of Veterinary Internal Medicine, 5th edn. Eds S J Ettinger and E C Feldman. WB Saunders, Philadelphia: Inzana, K D (2000). Peripheral nerve disorders. In: Textbook of Veterinary Internal Medicine, 5th edition. Eds S J Ettinger, E C Feldman. WB Saunders, Philadelphia: Further reading Feldman, E C and Nelson, R W (2004). In: Canine and Feline Endocrinology and Reproduction 3rd edn. WB Saunders: Lane D R and Cooper B (2003). Veterinary Nursing 3rd edn. Butterworth Heinemann, Oxford: / 12

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7 SVN taking a pulse as part of monitoring the vital signs in a patient. 7 / 12

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9 Some treatments available for hypothyroidism. PHOTO CREDIT: courtesy of Virbac (Soloxine) and Ceva Animal Health (Forthyron). 9 / 12

10 Some treatments available for hypothyroidism. PHOTO CREDIT: courtesy of Virbac (Soloxine) and Ceva Animal Health (Forthyron). Table 1. Normal TT4 levels* 10 / 12

11 GLOSSARY OF TERMS 11 / 12

12 Table 2. Myxoedema coma information 12 / 12 Powered by TCPDF (