Endocrine Clinical Flashcards
Possible reasons why congenital hyposomatotropism may occur?
Failure of Rathke’s pouch development (LHX3 mutation)
Hydrocephalus
When might congenital dwarfism be noted?
after 1-2 months, this is the period in which growth is more dependent on genetics.
Clinical features of congenital hyposomatotropism
Retained deciduous teeth Dull and dry hair coats Corneal oedema Weakness and lethargy Proportionate dwarfism
What conditions could falsely decrease IGF-1?
Liver disease Lymphoma Diabetes (newly diagnosed) Renal disease Body size
What is the pathophysiology of iatrogenic hypothyroidism?
The elevated thyroid hormones suppress TSH and so once this negative feedback is removed and the overactive thyroid cells are gone all that is left is atrophied cells. It takes time for these to grow again and start secreting T4.
What type of genetic inheritance pattern does congenital hypothyroidism show?
Autosomal recessive.
How can you tell the difference between congenital dyshormonogenesis and dysmorphogenesis in hypothyroidism?
Dyshormonogenesis may result in a goitre as TSH is high and the gland enlarges as there are cells there.
Dysmorphogenesis will not cause gland enlargement as the cells are just not there.
Key features of feline congenital hypothyroidism
Constipation/Megacolon
Goitre (with dyshormonogenesis)
Delayed growth plate closure –>Disproportionate dwarfism
Mental dullness (thyroid is important for neurological development)
Key features of adult onset hypothyroidism (cats)
Dry, dull, unkempt coat
Lethargy
Inappetence
Which gland should you compare the thyroid to when performing scintigraphy in dogs vs. cats?
Dog: parotid
Cat: zygomatic
Which feline breed has thyroid dyshormonogenesis been noted in?
Abysinnian
What is the pathophysiologic mechanism behind congenital hypothyroidism caused by dyshormonogenesis?
It is a problem with thyroid peroxidase.
What are the roles of the following in thyroid scintigraphy?
- Technetium
- Iodine - 131
- Iodine-123
- Perchlorate
- Tc = treated like iodine by the body so will be taken up by the thyroid gland
- I-131 = used for treatment
- I-123 = used for imaging
- Perchlorate = Competes with Iodide for NIS transporter. Administration in dyshormonogenesis will cause discharge of iodide ions out of the gland as they are not bound to thyroglobulin.
What proportion of hyperthyroid cats have unilateral disease?
Likely <1/3. >50% of cases have bilateral disease but often it is asymmetrical.
What proportion of hyperthyroid cats have erythrocytosis?
50%.
What is the most common historical clinical sign in hyperthyroidism?
Weight loss - affecting 88% of cases.
What is the percentage of normal cats that have a goitre?
70%
What are the main effects of thyroid hormones on the heart?
Increase heart rate, up regulates beta receptors
Inotropy - alters ion channel activity and enhances myosin activity
Hypertrophy - increases expression of myocardial proteins.
Reasons why thyroid hormone may cause PU/PD
• Thyroid hormone may lead to increased GFR
○ Increases NO activity in the renal cortex
○ Increases beta-adrenergic receptors
○ Both may lead to activation of RAAS
○ Afferent arteriolar resistance will decrease and increased hydrostatic pressure lead to an increased GFR
• Thyroid hormone effects on tubular function
○ Upregulation of chloride channels –> enhanced chloride reabsorption
○ Decreased Cl delivery to the macula densa triggers tubuloglomerular feedback and increases GFR
○ Enhanced Na/K-ATPase activity + Na/H exchange –> increased Na/Ca exchange –> increased calcium reabsorption
§ Mild hypercalcaemia is noted in people with HT
§ Hypocalcaemia has been noted in FHT
• Thyroid directly impacts on creatinine physiology
○ Increases GFR
○ Enhances tubular secretion of creatinine
○ Muscle loss
What is true about fT4, does it have good sensitivity or specificity?
It has a good sensitivity as it is unlikely that a normal cat would have a high fT4. However, it is not very specific and non-thyroidal illness and result in it being in or out of the normal range very easily.
What might the reason for a high fT4 and low TT4 be?
This pattern can be seen in euthyroid sick syndrome.
When might T3 suppression and TRH stimulation tests be useful in diagnosing FHT?
T3 - when basal testing has failure to confirm. In a normal cat T3 will suppress thyroid hormone production
TRH - used to diagnose occult hyperthyroidism. In HT TRH will have minimal effect.
What thyroid to salivary ratio is diagnostic for FHT in thyroid scintigraphy?
> 1.5
What is the therapeutic target of T4 for treatment of feline hyperthyroidism?
Middle to lower half of the RI whilst controlling clinical signs.
