PATHOLOGY - Hypothyroidism Flashcards
Describe the normal endocrine pathway that results in hormone release by the thyroid gland
The hypothalamus secreted thyrotrophin releasing hormone (TRH) which acts on the anterior pituitary gland to stimulate the secretion of thyroid stimulating hormone (TSH) which will act on the thyroid gland. The thyroid gland can produce T4, T3 and reverse T3 (which is metabolically inactive). T4 is secreted by the thyroid gland and most serum T4 is bound to serum proteins. Though the thyroid can produce T3, a considerable amount of T3 is derived from T4, with this process occuring mostly within the peripheral cells
Which thyroid hormone drives cellular metabolism?
T3 drives cellular metabolism
What are the functions of thyroid hormones?
Increase metabolic rate
Infant growth
Anagen phase of hair growth
Tonic regulatory effect on the cardiovascular, haematopoietic and orthpaedeic systems
What are the non-thyroidal influences on thyroid hormone production and secretion?
Age
Breed
Sex
Time of year
Time of day
Obesity
Starvation
Disease
Steroids
Anaesthesia
What is euthyroid sick syndrome?
Euthyroid sick syndrome is a normal decrease in thyroid hormone levels in response to starvation and disease in order to conserve energy and resources. In mild illness, serum T3 levels will decrease, and as starvation or disease progresses both T3 and T4 will decrease
(T/F) Euthyroid sick syndrome requires treatment with thyroid hormone replacement
FALSE. Euthyroid sick syndrome is a normal response to starvation and disease to conserve energy and resources and thus it would worsen the prognosis to alter this process
Which species is more prone to hypothyroidism?
Dogs
What are some of the potential causes of primary hypothyroidism?
Idiopathic atrophy
Congenital hypothyroidism
Goitre
Autoimmune lymphocytic thyroditis
What are the clinical signs of congenital hypothyroidism?
Hypothyroidism
Disproportionate dwarfism
Cretinism
Which dog breeds are predisposed to congenital hypothyroidism?
Boxer
Giant Schnauzer
What is the main secondary cause of hypothyroidism?
Anterior pitutary hypofunction (pituitary tumour)
What is the most common cause of hypothyroidism?
Autoimmune lymphocytic thyroditis
Describe the pathophysiology of hypothyroidism due to autoimmune lymphocytic thyroiditis
The thyroid tissue becomes damanged due to an autoimmune driven infiltration of lymphocytes. This damage to the thyroid will initally result in compensatory hypothyroidism which will increase thyroid stimulating hormone (TSH) production to increase and maintain T4 production. Eventually, this compensatory mechanism will be overwhelmed and the T4 will begin to decrease, resulting in early clinical hypothyroidism. Eventually, in advanced hypothyroidism, both the thyroid stimulating hormone (TSH) and T4 will decrease
What is a key marker of autoimmune lymphocytic thyroiditis?
Some patients will have thyroglobulin autoantibodies (TGAA)
What is the typicaly signalment for hypothyroidism?
Young to old age dogs
Which dog breeds are predisposed to hypothyroidism?
Golden Retriever
Labrador
Doberman
Boxer
How do you approach diagnosis of hypothyroidism?
History and clinical signs
Clinical examination
Haematology and biochemistry
Rule out other causes of clinical signs
Specific tests
Trial treatment
What are the potential clinical signs of hypothyroidism?
Lethargy
Weight gain
Cold intolerance
Hypothermia
Dermatological changes
Bradycardia
Pale mucous membranes
Female reproductive abnormalities
Neurological abnormalities
Ocular abnormalities
Which dermatological changes can be seen due to hypothyroidism?
Symmetrical but patchy, non-pruritic, alopecia
Seborrhea
Hyperpigmentation
Myxodema
What is seborrhea?
Seborrhea is where there is excessive flaking, scaling and itchy skin
What is myxoedema?
Myxoedema is the accumulation of glucosaminoglycans in the dermis which results in a puffy appearance of the face, thickened skin folds above the eyes along with a slight drooping of the upper eyelid, resulting in a characteristic ‘tragic facial expression’
Which female reproductive abnormalities can be seen with hypothyroidism?
Infertility
Abortion
Inappropriate lactation
Which neurological abnormalities can be seen with hypothyroidism?
Lower motor neurone disease
Laryngeal paralysis
Megaoesophagus
Peripheral vestibular disease
However it is challenging to determine if these conditions are due to the hypothyroidism or concurrent to the hypothyroidism
What is the main finding associated with hypothyroidism seen on biochemistry?
Hypercholesterolaemia
Which specific tests can be done to help to diagnose hypothyroidism?
Serum T4 levels
TSH assay
What is the normal range for serum T4 levels?
Normal serum T4 levels should be above 35nmol/l
What is the range for serum T4 levels indicative of hypothyroidism?
Serum T4 levels less than 14nmol/l are indicative of hypothyroidism
What are the disadvantages of using serum T4 levels when investigating hypothyroidism?
Measuring serum T4 levels has a low sensitivity as serum T4 levels can be normal is some hypothyroid causes. In these cases, thyroid function may appear normal despite the presence of clinical signs
Which results are indicative of hypothyroidism using a TSH assay?
Patients with hypothyroidism would have a low serum T4 but a high TSH level
What is the most important thing to be aware of when diagnosing hypothyroidism?
You have to diagnose hypothyroidism based on history, clinical signs, clinical pathology, specific tests, rule out other causes of clinical signs and assess their response to treatment
How do you treat hypothyroidism?
Sodium levothyroxine
What is sodium levothyroxine?
Sodium levothyroxine is a thyroid hormone replacement drug
What should you make owners aware of when beginning treatment for hypothyroidism?
Make it very clear to owners that treatment for hypothyroidism is a trial and if there is no improvement in clinical signs within two months, the medication will be stopped and further investigation will be performed
