Endocrinology (hypothyroidism) Flashcards
Hypothyroidism (11 cards)
Describe the epidemiology and the cause of congenital hypothyroidism
Naturally occurring hypothyroidism is an extremely uncommon disorder in cats
Most cases appear to be primary and are seen in kittens with an inborn error of metabolism
Although an inherited basis has been documented in some, many cases are observed as sporadic abnormalities
What are the clinical signs of congenital hypothyroidism
Disproportionate dwarfism (stunted growth, short legs, broad body and skull)
Delayed or incomplete eruption of permanent teeth
Mental dullness/retardation
Constipation
+/- Palpable goiter
Delayed physeal closure
Retention of undercoat and loss of primary hairs giving a short, soft coat
How can you make a diagnosis of hypothyroidism based on TSH
The TSH stimulation test is the preferred diagnostic test as this causes a greater proportionate rise in T4 and T3 levels in healthy cats
A TSH dose of 1 UI/cat IV stimulates maximum thyroid hormone concentrations 6-8 hours later with post-TSH T4 levels typically 2-4 times basal levels
Hypothyroid cats would be expected to have low basal T4 and fT4 levels which do not appreciably increase after TSH stimulation
The finding of an elevated TSH concentration is considered to be the most sensitive of the currently available tests for detection of iatrogenic or spontaneous hypothyroidism
How would you treat a cat with hypothyroidism and what is the prognosis
Therapy is with thyroid hormone supplementation (10-20 µg/kg daily)
The prognosis is variable
The earlier the condition is diagnosed the better the prognosis is
Explain the causes of adult-onset hypothyroidism
This is a sporadic natural disease in the cat
Iatrogenic hypothyroidism is seen with some frequency following radioiodine therapy or over-zealous administration of anti-thyroid drugs
- It is especially important that this possibility is investigated in azotemic cats
What are the clinical signs of hypothyroidism in the adult cat
Lethargy/apathy
Weight gain/obesity
Hypothermia/cold intolerance
Myxedema
Seborrhea sicca
Alopecia of the pinnae and/or mild alopecia of the general haircoat
Hyperkeratosis
PUPD with mild-moderate evidence of renal dysfunction (may be reversible with treatment)
Poor appetite
Bradycardia
Palpable goiter (common)
How can you make a diagnosis of feline hypothyroidism
The finding of a sub-normal TT4 is often due to “sick euthyroidism”
However, the clinician should be alerted to the possibility of acquired hypothyroidism if the cat is not otherwise unwell or has previously undergone radioiodine treatment
The finding of an elevated TSH is diagnostic
What are the two main categories of primary hypothyroidism and how can you differentiate them
Thyroid dyshormonogenesis
- it is a defect in any step of iodine uptake or thyroid hormone synthesis
- the low circulating TT4 concentrations lead to increased pituitary TSH secretion
- because the thyroid follicles remain intact, the high circulating TSH concentrations induce hyperplasia of the thyroid gland. Therefore, an enlarged thyroid gland (bilateral goiter) can sometimes be palpated
Thyroid dysgenesis
- it is a developmental defect of the thyroid gland, sometimes resulting from TSH receptor abnormalities
- it leads to hypoplasia or aplasia of the thyroid gland; this is a nongoitrous form
What are the main clinical differences between hypothyroid cats and hypothyroid dogs
Hypothyroid cats rarely develop generalized alopecia, a common sign in dogs
Cats may develop a poor appetite, a sign not reported in hypothyroid dogs
Kittens may present with severe constipation as a primary complaint
Could you make a difference between iatrogenic, congenital and adult onset hypothyroidism according to clinical signs
Most cats with adult-onset or iatrogenic hypothyroidism display mild or no clinical signs
- weight gain or obesity seems more common with iatrogenic hypothyroidism
- lethargy, dermatologic signs, poor appetite are more frequent with adult-onset hypothyroidism
Congenital hypothyroidism cats present with severe lethargy and constipation
Explain why dosing TSH in parallel with TT4 is important for a reliable diagnosis of hypothyroidism
A high TSH concentration confirms the disease is primary (i.e., located within the thyroid gland)
Healthy cats and cats with nonthyroidal illness generally have normal values for serum TSH
- a few cats with mild nonthyroidal illness, especially obese cats and cats with poorly controlled DM, will have high normal to mildly increased TSH values
In general, the finding of a low to low-normal TT4 or free T4 in combination with a high TSH greatly improves diagnostic sensitivity for hypothyroidism
