Hypothyroidism Flashcards
ACTH
adrenocorticotrophic hormone
ALP
alkaline phosphatase
ALT
alanine amonitransferase
cTSH
canine thyroid stimulating hormone
ED
equilibrium dialysis
FT3
free triiodothyronine
FT4
free thyroxine
GFR
glomerular filtration rate
GH
growth hormone
rT3
reverse triiodothyronine
SDMA
symmetric dimethylarginine
T3
triiodothyronine
T4
thyroxine
TSH
thyroid stimulating hormone
TT3
total triiodothyronine
TT4
total thyroxine
Euthyroidism meaning
- The physiological state characterized by normal serum levels of thyroid hormone
- Normal functioning of the thyroid gland
What is the most common endorine disorder in dogs?
Hypothyroidism
Three types of hypothyroidism and which one is most common?
Primary - most common (95%), others are very rare
Secondary
Tertiary
What happens in primary hypothyroidism?
Two options:
-Gradual destruction of the thyroid gland by autoimmune lymphocytic thyroiditis
or
-Idiopathic necrosis and atrophy
Two types of secondary hypothyroidism
Congenital
Acquired
What types of congenital hypothyroidism are there?
-Cretinism
-Together with GH deficiency - pituitary dwarfism
What is cretinism?
Conditition of severe physical and mental retardation due to iodine deficiency
Is caused by lack of thyroid hormones during early pregnancy
Can happen if diet is poor
What types of acquired hypothyroidism are there?
Pituitary neoplasia
Head trauma
Hypophysectomy
Lymphocytic adenohypophysitis
9 major functions of thyroid hormones
- Anabolism (in physiological levels) or catabolism (in excess)
- Increase weight (physiological) or decrease weight (in excess)
- Increase heart rate
- Increase blood pressure
- Increase body temperature
- Increase gut motility
- Increase CNS activity
- Are required for growth
- Are required for reproduction
Reproductive clinical signs of hypothyroidism
-Periparturient mortality (around and just before birth)
-Low birth weights
-Uterine inertia (total or partial absence of contractions)
-Irregular estrus
-Testicular atrophy
-Low libido/subfertility
What is myxoedema?
Swelling of skin - rounding of facial features
Hyaluronic acid
Myxedematous coma - rare, life-threatening clinical condition that represents severe hypothyroidism with physiologic decompensation
–> hypothermia, depressed mental state, hypotension, bradycardia etc.
Sequence of diagnostic tests (4)
- Clinical examination
- Hematology
- Biochemistry panel
- Functional tests
Hematology findings in hypothyroidism
Normocytic normochromic nonregenerative anemia
Most important biochemistry finding(s)
Hypercholesteremia (because this is rare in other conditions)
Other findings:
-Hypertriglyceridemia
-Increased ALT, ALP
-Increased creatine kinase
Do cats ever have hypothyroidism?
Most common form is iatrogenic hypothyroidism after treatment for hyperthyroidism.
Naturally occuring hypothyroidism is possible, but extremely rare.
What kind of dogs (age, size) usually get hypothyroidism?
4-10 years old
Mid- to large-size breeds
Rare in toy and miniature breeds
What is the most commonly performed functional test?
Total T4 concentration is often used as a initial screening test but it can be problematic.
A normal TT4 value can be used to rule out hypothyroidism.
A low TT4-value is not diagnostic and you should do more tests.
For example, TT4 might also be low in “Euthyroid sick” syndrome.
(Except in cats TT4 is very good and the only test you need!)
What is Euthyroid sick syndrome?
A condition in which serum levels of thyroid hormones are low in patients who have nonthyroidal systemic illness but who are actually euthyroid. Diagnosis is based on excluding hypothyroidism.
Some examples: starvation, trauma, myocardial infaction, chronic kidney disease, diabetic ketoacidosis, cirrhosis, sepsis, drugs…
Name some common drugs that can affect thyroid function tests in dogs
-Glucocorticoids - decrease TT4 and T4
-NSAIDs - decrease TT4
-Barbiturates - decrease TT4 and T4, mildly incease cTSH
-Clomipramine - Decrease TT4, T4, TT3
-Thyroid hormone replacement therapy - Decreases cTSH through negative feedback loop
When can TSH measurement be useful?
Helps differentiate low T4 of hypothyroidism from other causes
TSH should not be interpreted alone!
Can be within reference intercal in a high proportion of hypothyroid dogs
Some advantages and disadvantages of free T4 measurement
Less affected by non-thyroidal illness and drug therapies than TT4.
Decreased values are more specific for hypothyroidism than TT4.
More expensive and complicated to measure than TT4.
Recommendation for routine testing of thyroid function:
- Measure TT4 and TSH together in all cases.
- If TT4 is low and TSH is high, diagnosis is nearly certain.
- Take free T4 if one of these conditions is present:
a) TT4 and TSH are inconclusive
b) antithyroglobuline antibodies are suspected
c) nonthyroidal illness exists - Be aware of the effects that nonthyroidal illness and medications have on thyroid function tests
- Appropriate response to levothyroxine administration should confirm diagnosis
+ Clinical signs of hypothyroidism
Importance of autoantibodies
They may interfere with the determination of thyroid hormone concentrations.
–> usually false negative, sometimes, false positives
They are useful for diagnosis of autimmune thyroiditis but are not measures of thyroid functions.
In complicated cases, what additional diagnostic tests might you do?
-TSH stimulation test
-TRH stimulation test
-Scintigraphy
-Histopathology
-Ultrasound
What’s the TSH stimulation test?
Evaluates the reserve function of the thyroid gland.
Is considered to be more reliable than any other single individual test for canine hypothyroidism.
What’s the TRH stimulation test?
Idea is that TRH stimulates release of TSH which stimulates production of T4 and T3.
Can be used to differentiate secondary and tertiary hypothyroidism, or between primary and nonthyroidal illness.
What about T3?
T3 is the metabolically active form of T4.
Only 50% is secreted by thyroid gland, other 50% is formed by deiodination of T4 in peripheral tissues.
In general, does not really accurately reflect thyroid function, not a useful test.
What is the treatment for hypothyroidism?
Lifelong treatment with levothyroxine.
Prognosis in hypothyroidism?
Excellent in most cases.
Individual variation.
Check-up every 1/2 year.
With myxedema coma, prognosis is worse.
In case of congenital version, dog may have residual mental retardation and skeletal abnormalities.
What are 4 learning outcomes from the lecture that the teacher listed?
- Diagnosis can be very challenging in some dogs.
- Hypothyroidism should be on the differencial list of many sick dogs.
- Euthyroid sick syndrome is clinically very important.
- Autoantibodies may interfere with the determination of thyroid hormone concentrations.
