The veterinary nurse’s guide to thyroid disorders
How much do you do with your hyperthyroid and hypothyroid patients?
Thyroid disorders are some of the most common canine and feline endocrine diseases that we see in practice, and we have a lot of opportunities to improve the care these patients receive, both in the clinic and at home.
Join me today as we talk all about hyper and hypothyroidism, why they occur, the signs they cause and why, and how we can treat and nurse these patients to the highest standard.
Hypothyroidism
Hypothyroidism is a common endocrine disease affecting senior dogs, and resulting in the inadequate secretion of the 2 thyroid hormones T3 and T4.
The condition is either primary or secondary - primary hypothyroidism is more common, and is seen either due to immune-mediated thyroiditis (where thyroid tissue is replaced progressively with fibrous tissue), or idiopathic atrophy of the thyroid gland.
Secondary hypoparathyroidism is rarer and is caused by malformation or neoplasia of the pituitary gland causing low secretion of thyroid stimulating hormone (TSH).
Hypothyroidism is commonly seen in Golden Retrievers, Dobermans, Irish setters, Great Danes, Airedales, English sheepdogs, Schnauzers, Spaniels, Poodles and Boxers.
Clinical Signs
Clinical signs are usually non-specific, varied, and gradual in onset, affecting a variety of body systems:
Metabolic: lethargy, dullness, weight gain, cold intolerance
Dermatologic: alopecia, seborrhoea, dry/brittle coat, changes in hair coat and colour, pyoderma
Neuromuscular: weakness, ataxia, vestibular signs, facial nerve paralysis, seizures
Cardiovascular: bradycardia, cardiac arrhythmias
Ocular: corneal lipid deposits, dry eye, corneal ulceration, uveitis
Gastrointestinal: diarrhoea, constipation
Haematology: anaemia, coagulopathy, hyperlipidaemia
Diagnosis
Hypothyroidism is diagnosed by measuring thyroid hormone and TSH levels in addition to routine biochemistry and haematology +/- urine analysis.
Total thyroxine (T4) levels are commonly measured in patients suspected to have thyroid disorders. This is the total circulating T4 level and includes both T4 that is freely available in the bloodstream and protein-bound T4.
In some cases, total T4 levels alone are not sufficient for accurate diagnosis. This is because T4 levels are altered by several medications (e.g. NSAIDs, phenobarbital, furosemide and steroids) and non-thyroidal illnesses (such as renal, hepatic and cardiac disease). This can result in falsely low total T4 results in a non-hypothyroid patient.
To combat this, we also measure TSH levels, and, in some cases, the freely-available T4 (non-protein-bound T4) in the bloodstream. A high TSH with a low T4 is suggestive of primary hypothyroidism - since the pituitary will continue trying to stimulate the thyroids to release thyroid hormones.
On other bloodwork, elevated cholesterol and triglycerides are commonly seen on a fasted sample, since these patients often have hyperlipidaemia.
Treatment and Nursing Care
Treatment is achieved with synthetic thyroxine and monitored with regular T4 bloods, ideally taken at the same time of day each time (around 4-6 hours post-pill for peak T4 levels). Client education is an important part of long-term care of these patients, and the veterinary nurse is often involved in this process. Discussion of administering medications, advising on follow-up appointments and collecting repeated blood samples are all key nursing roles in the hypothyroid patient.
Hyperthyroidism
Feline hyperthyroidism is a multisystemic disorder resulting from the excessive secretion of thyroid hormones. It is the most common endocrinopathy in cats, and is commonly diagnosed in middle-aged to senior cats (with a median age of 13 years).
97-99% of cases are caused by a benign mass on the thyroid gland(s) called a thyroid adenoma, which secretes thyroid hormones. The remaining 1-3% are due to a malignant thyroid carcinoma.
Additionally, over 70% of cases have bilateral disease, and ectopic thyroid tissue - thyroid tissue in other locations within the body, such as the mediastinum - is also common.
The exact cause is unknown but it is suspected to be multifactorial - including diet, environmental and genetic causes.
Clinical Signs
Clinical signs of hyperthyroidism are associated with the increased metabolic rate we see due to excessive thyroid hormone levels. These include weight loss, polyphagia, tachycardia, heart murmurs +/- arrhythmias, PU/PD, vomiting and/or diarrhoea, poor hair/coat quality and behavioural changes.
Diagnostics
A variety of diagnostic tests are performed in the hyperthyroid patient. These include biochemistry and haematology, thyroid testing, blood pressure measurement and echocardiography.
Biochemistry and haematology often shows increases in ALP and ALT, and increases in red blood cells, neutrophils and lymphocytes may also be seen.
Total T4 levels may be increased in the hyperthyroid cat, or, if non-thyroidal illness is also present (or if the patient is receiving medications which affect T4 levels), a falsely normal result may be seen. The diagnosis is confirmed by measuring free T4 levels, since these are less affected by non-thyroidal illness.
Other diagnostics which may be performed in the hyperthyroid cat include blood pressure measurement, which is very important since systemic hypertension is common in hyperthyroid cats, in addition to echocardiography if a heart murmur is present.
Less common diagnostic tests include nuclear scintigraphy, which is generally only performed in large specialist centres. This imaging technique involves administering an injection of technetium-99, then imaging the patient with a gamma camera. The technetium concentrates in thyroid tissue throughout the body, allowing enlarged thyroids and ectopic thyroid tissue to be visualised.
Treatment
Hyperthyroidism is treated either medically, surgically, with radioactive iodine therapy or with dietary management.
Medical Management
Medications for hyperthyroidism inhibit the synthesis of thyroid hormones. There are 2 main options for medication - methimazole (felimazole, thyronorm, etc) and carbimazole (vidalta). Methimazole requires twice-daily administration, whereas carbimazole only needs to be given once a day. This is because carbimazole is a methimazole prodrug - it is converted to methimazole in the body.
Medical management is also indicated prior to surgery or treatment with radioactive iodine, to stabilise the patient, allow accurate assessment of renal function, and improve their suitability for anaesthesia.
Surgical Management
Thyroidectomy (surgical removal of the thyroid gland) is performed less frequently since many hyperthyroid patients have bilateral disease and ectopic thyroid tissue which cannot be removed surgically.
Surgery carries a risk of damaging or removing the surrounding parathyroid glands, so care must be taken to prevent this. Additionally, careful monitoring for signs of hypocalcaemia should take place following surgery, in addition to monitoring calcium levels at regular intervals.
Signs of hypocalcaemia include:
Twitching
Facial rubbing/pruritis
Behavioural changes
Seizures
Horners syndrome may also be seen postoperatively if there is damage to the nerves near the surgical site, so monitoring pupil size and symmetry in these patients is recommended following surgery.
Other postoperative care considerations include pain assessment and administration of analgesia, nutrition and fluid balance. Care should be taken when obtaining blood samples post-operatively, since the surgical site sits near the jugular vein, and extending the neck may be painful or poorly tolerated. Medial saphenous sampling works quite well for these patients postoperatively, since we are far away from the surgical site.
Radioactive Iodine Therapy
Radioactive iodine therapy is a simple and effective method of treating hyperthyroidism in patients who don’t tolerate long-term medication. In most cases, a single dose of radioactive iodine is sufficient to destroy all hyperfunctioning thyroid tissue whilst preserving the surrounding hypofunctional and parathyroid tissue.
It is achieved with an injection of a radioactive isotope of iodine (I-131) which is given either IV or SC. The I-131 then accumulates within thyroid tissue where it releases gamma rays and beta particles, which penetrate up to 2mm, destroying the local hyperfunctioning tissue, but not any neighbouring tissues.
The main consideration with I-131 is that, following injection, the patient will be radioactive and will require careful management in the hospital until the half-life of the I-131 is exceeded.
This means that during hospitalisation, contact with the cat should be minimised, and clinical waste should be stored in a concrete store until the radiation has decayed.
Once the cat is discharged, for the first 1-3 weeks, clients should:
Minimise close contact with their cat
Prevent pregnant people or children from handling the cat or the cat’s waste
Flush their cat’s waste down the toilet
Iodine Restricted Diets
More recently, treatment with an iodine restricted prescription diet has become available. This replaces long-term medical management; since thyroid hormones are synthesised from iodine in the diet, restricting dietary iodine prevents excessive synthesis of these hormones.
This diet should be fed exclusively, since other diets will have differing iodine concentrations. Care should be taken in multicat households to prevent mixing of diets.
Patients being treated with an iodine-restricted diet should receive repeat appointments and blood tests as for any patient on long term treatment (every 3-6 months). Patients should be titrated off of medications where appropriate and transitioned to the diet over several weeks.
Canine Thyroid Tumours
Thyroid tumours account for 1-2% of all canine neoplasias; 70-100% of them are malignant, and around 30-60% have metastasized at time of diagnosis/presentation.
These tumours can result in either hypothyroidism or hyperthyroidism - up to 10% of cases are hyperthyroid, as the tumour is functional and secretes thyroid hormones. In other cases, the tumour destroys functioning thyroid tissue, resulting in a reduction in thyroid hormone secretion.
Thyroid tumours mostly affect older dogs >10 years and are most commonly seen in Beagles, Golden Retrievers and Boxers.
Clinical Signs
Patients commonly present with a ventral neck mass, and may also have trouble swallowing, regurgitation and coughing. Clients may also notice a change in the patient’s bark/vocalisation. Tachypnoea and/or dyspnoea may also be seen.
If the patient has a functional thyroid tumour (causing hyperthyroidism), signs such as PU/PD, polyphagia and behavioural changes may be seen, alongside the other clinical signs of hyperthyroidism (such as poor coat quality, weight loss and vomiting/diarrhoea).
Diagnostics
Diagnostic tests include routine bloodwork, thyroid function (T4/TSH) testing, advanced imaging and sampling.
If a patient has a functional thyroid tumour, increased T4 levels may be seen. Alternatively, if the patient’s tumour has destroyed functional thyroid tissue, a low T4 and high TSH may be present.
Diagnostic imaging ideally would include computed tomography (CT) of the cervical area, thorax and abdomen (to screen for metastasis) with the administration of a radiographic contrast medium. If this is not available then thoracic radiography and cervical and abdominal ultrasound should be considered.
If available, nuclear scintigraphy may also be used to assess the invasiveness of the tumour and whether local lymph nodes are involved.
Definitive diagnosis is achieved via fine needle aspirate or biopsy of the lesion and cytology/histopathology.
Treatment
Treatment depends on the size and invasiveness of the tumour present.
Surgical excision is indicated for small, mobile masses, whereas masses not suitable for complete surgical excision are generally treated with radiation therapy.
I-131 (radioactive iodine therapy) may be used either as a standalone treatment or as an adjunct to surgery to improve survival times.
Chemotherapy may also be considered; common agents include doxorubicin, cisplatin, toceranib or metronomic chemotherapy. This is generally recommended in patients with a high risk of metastasis following surgery or radiation therapy.
As you can see, we encounter a wide variety of thyroid disorders in practice, and they have significant effects on our patients. By involving ourselves in the long-term care of these patients, we nurses can make a significant difference to their treatment and quality of life. There are a variety of things we should be discussing with these patients, including:
Administering medications
Maintaining a normal weight and managing appetite
Adjusting diets and feeding (especially if using an iodine-restricted diet)
Mobility, exercise and lifestyle adjustments
Coat quality
Monitoring at home
Repeat appointments and follow-up tests/care
Other senior pet considerations
Quality of life
Do you regularly see thyroid patients as a nurse in your clinic? If not, why not?
If this is something you’d like to do more, I’m going to be releasing an exciting training in early 2022 that you won’t want to miss - pop your email address down here to be the first to hear more about it!
References:
Merrill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. Iowa: Wiley-Blackwell.
