Endocrine One Flashcards
Where is all circulating T4 produced?
Produced by the thyroid glands
What are the two sources of T3 production?
- Thyroid glands 2. Rest is generated from T4 around the body
What is is the precursor of thyroid hormones?
Thyroglobulin
Does T3 and T4 normally travel around the body by itself?
No the majority is bound to protein
T4 and T3 are both bound to proteins within the blood of dogs and cats. What are the proteins in dogs and what are they in cats?
Dogs; T4 is bound to thyroid hormone-binding globulin (TBG), transthyretin, albumin + apolipoproteins Cats: only prealbumin
What are the three main actions of thyroid hormone?
- Development 2. Growth 3. Metabolism
What are the effects of thyroid hormone on metabolism?
- Increased basal metabolic rate - increased HR, cardiac contractility, promote vasodilation
- Effects on lipid metabolism - stimulate fat mobilisation, enhance oxidation of fatty acids
- Effects on carbohydrate metabolism - insulin dependant entry into cells, increased gluconeogenesis + glycogenolysis
Fill in the following:
i) Name location 1 and 3
ii) Pre-hormone 12
iii) The rest are hormones
T3 and T4 can only exert their negative feedback and metabolic actions when they are in free/protein bound form:
Select the appropriate answer
Must be in free form
Comment on the frequency of primary hyperthyroidism and provide three examples of conditions that could cause this:
Primary hypothyrodism = caused by thyroid gland disease (++++ common)
ACQUIRED:
- Iodine deficiency = goitre (can be congenital or acquired)
- Lymphocytic throiditis
- Idiopathic follicular atrophy
For secondary and tertiary hypothyrodism:
i) Comment on the frequency of these conditions
ii) State the location where the defect occurs
iii) Provide a specific example of a condition that could lead to these defects
i) Both of these conditions are very rare
ii) Secondary hypothyroidism = caused by pituaritary disease whereas tertiary hypothyrodism = caused by hypothalamus
iii) Examples of secondary = pituitary neoplasia and surgical hypophysectomy, tertiary hypothyrodism = tumour entering into the hypothalamus
What are goitrogens?
Goitrogens = compounds that cause hyperplastic goitre (access to cauliflower or brocoli)
Breifly explain the pathogenesis of lymphocytic immune mediated thyroiditis:
Developement of canine hypothyroidism is associated with a loss of self tolerance in lymphocytes
What are the four main clinicopathological changes that are seen with hypothyroidism in dogs?
- Mild normochromic, normocytic, non-regenerative anamia - reduced metabolic rate = decreased need for O2 in peripheral tissue - decreased O2 leads to decreased EPO production
- Hyperlipaemia - accumulation of lipids in the hepatocytes
- Increased hepatobiliary enzymes due to hepatic lipidosis
- Increased fructoasamine - albumin binding to glucose - occurs due to decreased protein turnover
- Increased CK - hypothyroidism induced myopathy
What is euthyroid sick syndrome?
The condition wherby a systemic conditon outside of thyroid gland creates hypothyroxaemia
Common disorders associated with ESS: Inflammatory, Neoplastic, Metabolic (e.g. renal failure), Endocrine (hyperadrenocorticism +++ common)
What are the mechanisms of euthyroid sick syndrome?
- Decreased protein bound T4 decreased concentration/affinity for binding proteins
- Inhibition of TSH secretion
- Inhibition of T4 production
Why does measurement of serum T3 have limited value is assesing whether or not an animal has hypothyroidism?
Significant overlap in serum T3 concentration between healthy, hypothyroid and euthyroid dogs –> limited diagnostic value
Why might serum tT3 concentrations be used?
These tests can be used to assess owner compliance
Explain what the implication of high sensitivity and low specificity is when measuring total T4:
If T4 is high hypothyrodism is unlikely. Low specificity - if T4 is low hypothyroidism in not confirmed
What breed is total T4 generally low in?
Greyhounds
State three conditions that can result in reduced total t4 in euthyroid dogs:
- Age
- Breed
- Durgs administration (particularly NSAID’s + corticosteroids)
Evaluate the sensitivity and specificity of free T4:
High specificity - less affected by extra-thyroidal factors/T4AA
Medium- high sensitivity - concentration of fT4 remains on low end of RI in up to 20% of cases
Free T4 test: More expensive test, Could be useful when tT4 cocentration could be low due to extra-thyroidal factors
Name the likely condition in each of the following circumstances:
i) Increases TSH and decreased tT4
ii) Increased TSH and increased tT4
iii) Decreased TSH
i) Hypothyrodism
ii) Hyperthyrodism
iii) Not primary hypothyroidism, Secondary hypothyroidism, ESS
