Hypothyroidism Flashcards
what are the effects of thyroid hormones (4)
- metabolism: increases metabolic rate
- infant growth: cretinism
- hair growth: activates anagen
- others: cardiovascular, hemapoietic, orthopaedic
how does the thyroid gland produce hormones
Consists of follicles which are lined by follicular cells which are full of colloid
Colloid is iodinated thyroglobulin
Thyroglobulin is made by the follicular cells using large numbers of tyrosine molecules that undergo iodination using the enzyme thyroid peroxidase
Thyroglobulin is broke down in the follicular cell and broken down through proteolysis to produce T4 — T3
what is the difference between free T4 and total T4
In circulation there is a small amount of free thyroxine but about 99% of blood T4 is bound to protein
Can cross through membranes and get into cells and is converted to T3 —> which will have the effect in the cell
how does the feedback loop of T4 work
Pituitary gland:
Produces TSH which stimulates the thyroid to produce T4 —> negative feedback loop
Excessive T4 is removed into rT3 if it needs to lower metabolic rate
Hypothalamus:
Releases TRH —> thyrotropin releasing hormone which stimulates pituitary to release TSH
Classic negative feedback loop
Higher centres of brain control both the pituitary and hypothalamus
Cortisol, IL-6, TNF-alpha, etc can influence the production of TRH, TSH and T
what are the most common causes of hypothyroidism
Primary: most common form —> results from problems within the thyroid gland, usually destruction Lymphocytic thyroiditis (>90%) (immune-mediated destruction)
Will see increased levels of thyroid hormone autoantibodies in early stage of disease then will eventually see a decrease in T4 in the advanced stage
Idiopathic atrophy
Congenital (rare)
what are non-thyroidal influences on thyroid hormones (3)
- physiological: age, breed, sex, time of day, time of year, obesity, starvation
- therapeutic: steroids, potentiated sulphonamides, diuretics, sedatives/anaesthetics
- pathological: sick euthyroid syndrome
what other factors can cause non-thyroidal illness (4)
- starvation
- surgery/anesthesia
- neoplasia
- other endocrinopathies
what are the causes of primary hypothyroidism (3)
- lymphcytic thyroiditis
- idiopathic atrophy
- congenital
what breeds are predisposed to congenital hypothyroidism
Giant Schnauzer, Boxer
what are the signs of congenital hypothyroidism
Disproportionate dwarfism
Cretinism
Hypothyroidism
what breeds are predisposed to primary adult hypothyroidism
golden retriever, lab, doberman, boxer
what are the stages of diagnosing hypothyroidism
history
clinical exam
routine blood tests
specific tests
trial therapy
what are the clinical signs of hypothyroidism
Mental dullness
Lethargy
Exercise intolerance
Propensity to gain weight without corresponding increase in appetite or food intake
Cold intolerance
what are the dermatological clinical signs of hypothyroidism
Classic cutaneous signs include:
Bilaterally symmetric, non-pruritic truncal alopecia that tends to spare the head and the extremities
Alopecia may be local or generalized and symmetric or asymmetric
Seborrhea and pyoderma
Dull hair coat, dry and easily epilated
Hair regrowth is slow
Hyperkeratosis leads to development of scales and dandruff
Hyperpigmentation
Myxedema (skin thickening)
Predominantly in the forehead and face of dogs resulting in rounding of the temporal region
Puffiness and thickening of the facial skin folds and drooping of upper eyelids
“Tragic facial expression”
what are the cardiovascular changes seen with hypothyroidism
Bradycardia: small QRS complexes, weak apex beat
Arrhythmias
what are the hematological changes seen
pale mucus membranes
non-regenerative normochromic anemia
what reproductive abnormalities are seen
Persistent anestrus
Abortion
Inappropriate galactorrhea
what neurological abnormalities are seen
Lower motor neurone disease
Peripheral vestibular syndrome
Laryngeal paralysis
Common in old labs as well as hypothyroidism
Megaesophagus
what occular abnormalities are seen
Lipid effusion into aqueous humour
Corneal lipidosis
Chronic uveitis
Glaucoma (secondary)
Keratoconjunctivitis sica
what are the common clinical manifestations
lethargy
weight gain
alopecia
pyoderma
seborrhea
what are the uncommon clinical manifestations
neuromuscular disoders
female infertility
myxoedema
ocular disorders
congenital disease
what are the clinical pathology can help confirm the diagnosis
cholesterol
triglycerides
PCV
CK
thyroid hormones
assays for TSH
assay for anti-thyroid antibodies
what are the normal T4 levels
>35 nmol/l
what is the grey zone of T4 levels
14-35 nmol/l
