Thyroid glands Flashcards
what are the two types of endocrine cells found in the thyroid?
follicular
parafollicular (medullary/C) cells
what do follicular cells of the thyroid produce?
thyroid hormones
what do parafollicular/medullary cells of the thyroid produce?
calcitonin
what is thyroid hormone secretion regulated by?
hypothalamic-pituitary-thyroid axis
what does the hypothalamus release as part of the hypothalamic-pituitary-thyroid axis?
thyrotropin releasing hormone (TRH)
what does thyroid releasing hormone act on?
pituitary gland
what does the pituitary gland release as part of the hypothalamic-pituitary-thyroid axis?
thyroid stimulating hormone (TSH)
what does TSH act on?
thyroid gland
what does the thyroid gland produce as part of the hypothalamic-pituitary-thyroid axis?
T4 (thyroxine) and T3
what effect does T4 nd T3 have on the pituitary gland and hypothalamus?
negative feedback to prevent the release of TRH and TSH
what happens to T4 in peripheral tissue?
synthesised to T3
does the thyroid produce more T3 or T4?
T4
is T4 or T3 more biologically active?
T3
how is most T3 and T4 transported?
bound to plasma proteins (globulins)
what is the effect of thyroid hormones on metabolism?
increased metabolism
what are the functions of thyroid hormones?
increases metabolic rate of tissue
positive inotrope/chronotrope
increase number/affinity of beta-adrenergic receptor
enhance response to catecholamines
catabolic effect of muscle and adipose tissue
stimulate erythropoiesis
regulate cholesterol
what effect do thyroid hormones have on the heart?
positive inotrope and chronotrope
what is hypothyroidism?
clinical syndrome resulting from deficiency of T3 and T4
how does thyroid hormone increasing the number/affinity of beta-adrenergic receptors and enhancing their response to catecholamines effect the heart?
increases heart rate
what animals is hypothyroidism most commonly seen in?
middle/older aged dogs
what causes congenital hypothyroidism?
thyroid hypoplasia, aplasia or dyshormonogenesis
how can the pituitary gland cause congenital hypothyroidism?
panhypopituitarism causes TSH deficiency leading to thyroid hormone deficiency
what breed is panhypopituitarism seen in?
German shepherds
is acquired or congenital hypothyroidism more common?
acquired
what are the two types of acquired hypothyroidism?
primary (due to thyroid gland)
secondary (due to pituitary gland)
tertiary (due to hypothalamus)
what are some causes of primary hypothyroidism?
lymphocytic thyroiditis
idiopathic atrophy
thyroid neoplasia
what type of disease is lymphocytic thyroiditis?
immune-mediated
what are the clinical signs of hypothyroidism?
metabolic - lethargy, obesity, exercise intolerance, cold intolerance
dermatological - thinning/alopecia, poor coat quality, skin hyperpigmentation, pyoderma
diarrhoea, bradycardia, altered oestrus cycle, neuropathy
can haematology and biochemistry diagnose hypothyroidism?
no - may may point towards it or help to rule in/out other disease
what will be the features on haematology of a dog with hypothyroidism?
mild normocytic normochromic non-regenerative anaemia
what changes will be seen on biochemistry of a dog with hypothyroidism?
hypertriglyceridaemia
hypercholesterolaemia
mildly increased CK
mildly increased ALP and ALT
fructosamine increase
how does the serum of a dog that has hypothyroidism appear?
milky colour due to hypertriglyceridaemia
why are hypothyroidism dogs hypertriglyceridaemic?
because they have less lipid metabolism so it remains in the blood
why does ALP and ALT mildly increase with hypothyroidism?
lipid storage in the liver due to decreased metabolism causing damage
why does CK mildly increase in hypothyroidism cases?
muscle catabolism
how is required to diagnose hypothyroidism?
thyroid hormone testing
what ways can thyroid hormone be measured?
total T4
free T4
total T3
TSH
what test results confirm hypothyroidism?
increased TSH and decreased total/free T4 (thyroxine)
what dynamic function tests are available for thyroid hormone testing?
TRH or TSH stimulation
what two things have to be measure to confirm hypothyroidism?
TSH and T4
what is the most common test for hypothyroidism?
total T4
what are some possible reasons for false positives when using total T4 to test for hypothyroidism?
can be low due to non-thyroidal illness
some drugs can cause it to be low
older dogs can have low T4
what breeds have low-normal values of total T4?
greyhounds
what happens to free T4 if the sample is left out?
concentration significantly increases if left at room temperature for prolonged periods
what can cause a false increased in free T4?
severe lipaemia
samples stored at room temperature
how will TSH be effected in hypothyroidism?
increased (sometimes normal)
what can cause increases in TSH other than hypothyroidism?
recovery from non-thyroid illness
use of sulphonamides
what should TSH always be measured with?
T4
what can thyroid hormone autoantibodies be used to provide evidence for?
provide evidence of immune reaction to the thyroid gland (doesn’t diagnose hypothyroidism)
how can thyroid hormone autoantibodies effect total T4/T3?
can cause increases in total T3 or T4 because tests can misinterpret the autoantibodies as T3 or T4
how does prednisolone effect thyroid hormones?
causes decrease total and free T4
normal or decreased TSH
how does phenobarbital effect thyroid hormones?
decrease total and free T4
increased TSH
how does sulphonamide effect thyroid hormone?
decreased total and free T4
increased TSH
what drugs cause hypothyroidism?
sulphonamides
how do sulphonamides cause hypothyroidism?
inhibits thyroid peroxidase which is the enzyme used to release thyroid hormone from the thyroid
how is hypothyroidism treated?
levothyroxine (thyroid hormone supplementation)
how long is the treatment cause of levothyroxine for hypothyroidism?
life long
why does care need to be taken when treating hypothyroidism in cases that have concurrent cardiac problems?
treating will cause increased heart rate which could compromise the heart
how is the response to hypothyroidism treatment monitored?
look at clinical signs
measure total T4 (4-6 hours post treatment)
how long should it take the metabolic signs of hypothyroidism to resolve after treatment?
couple of weeks
what are some common complications of hypothyroidism?
neurological - generalised myopathy, megaoesophagus, laryngeal paralysis
hyperlipidaemia, atherosclerosis
myxoedema coma
is hypothyroidism more common in cats or dogs?
dogs (rare in cats)
what is the most common cause of hypothyroidism in cats?
iatrogenic - over treatment of hyperthyroidism, bilateral thyroidectomy, radioactive iodine treatment
how is hypothyroidism of cats diagnosed?
canine TSH
how do most cats present with hypothyroidism? (very uncommon)
goitre
