Internal Medicine: Endocrinology: Thyroid Flashcards
- Where is the thyroid found in cats and dogs?
- What are the two types of endocrine cells?
- What is required for hormone synthesis?
- Comprised of two distinct lobes on either side of the trachea
- Follicular cells- thyroid, Parafollicular or C cells- calcitonin
- Adequate dietary iodine is required
- Describe the functional unit (follicle) of the thyroid
- What hormones do the thyroids secrete and what are their function?
- What produces calcitonin and what is its function?
- A sphere of cells with a lumen containing a clear proteinaceous colloid- containing primarily thyroglobulin- a large glycoprotein dimer that serves as reservoir for thyroid. Parafollicular C cells lie in the interstitium between follicles
- T3 and T4- increase metabolic rate
- Thyroid C cell- reduced plasma calcium concnetration by binding to an inhibiting osteoblasts- stopping resorption- calcitonin and parathyroid decrease renal tubular reabsorption of phosphorus
How is thyroid secretion regulated?
- Hypothalamic- pituitary- thyroid axis
- Low plasma concentrations of thyroid hormones stimulates secretion of thyrotropin-releasing hormone (TRH)- by the hypothalamus and TSH by pituitary thyrotrophs
- Allmost all circulating T3/4 is bound to transthyrectin (protein bound)
How does thyroid enter and act on cells?
- T3 and T4 enter cells by transporter proteins
- T3 enters faster and more rapid onset and more potent
- Thyroid binds to receptors in the nuclei
- Influences the expression of genes coding for regulatory enzymes
- Thyroid hormone is also believed to have some non-genomic effects
What are the functions of thyroid hormones?
- Increase the metabolic rate- O2 consumption of most tissues
- Have positive inotropic and chronotropic effects on the heart
- Increase the number and affinity of beta-adrenergic receptors and enhance the response to catecholamines
- Have catabolic effects on muscle and adipose tissue
- Stimulate erythropoiesis and regulate cholesterol synthesis and degradation
- What is hypothyroidism?
- What can be the different causes?
- What are the predisposed breeds?
- Clinical syndrome resulting from a deficiency of T3/4
- Any abnormality in the hypothalamic-pituitary-thyroid axis- primary, secondary or tertiary (thyroid, pituitary, hypothalamus)- congenital or acquired
- English cocker spaniels, golden retrievers, dobermans
- What causes congenital hypothyroidism?
- How does it often clinically present?
- Thyroid hypoplasia, aplasia or dyshormonogenesis
- Disproportionate dwarfism
What are the most common causes of primary and secondary/tertiary (acquired) hypothyroidism?
Primary- most common- destructive
* Lymphocytic thyroiditis
* Idiopathic atrophy
* Aggressive/extensive thyroid neoplasia
Secondary/tertiary
* Defect in the pituitary or hypothalamus
What are the clinical signs of hypothyroidism?
How can congenital hypothyroidism appear?
Relate to metabolic and dermatological abnormalities
* Lethargy
* Hair thinning or alopecia
* Dry/poor quality coat
* Obesity/weight gain
* Excercise intolerance
Congenital
* Evident at 8 weeks of age
* Disproportionately wide skills
* Macroglossia
* Delayed dental eruption
* Square trunk and short limbs
* Possibly goitre
How is hypothyroidims in dogs diagnosed?
Thyroid hormone required for diagnosis
* Measurment of total/free T3/4 and TSH
* Antibody measurment (TgAA, T4AA, T3AA)
* Dynamic function tests
Haematology and biochem will not diagnose hypothyroidism
* Mild normocytic normochromic non-regenerative anaemia
* Hypercholsteraemia
What are the pros and cons of total thyroxine (TT4) test?
- Cheap and readily available
- Sensitive (low false negative)
- Not specific (high false positives)
TT4 is decreases in many non-thyroidal illnesses, and by many drugs
Some dogs- greyhounds and sighthounds have low normal values- use tt3
What are the pros cons and considerations of free thyroxine (FT4)?
- Measured by equilibirum dialysis
- Concentration significantly increases if stored at room temp for prolonged periods
- Usually normal in animals with low tT4 due to non-thyroidal illness
- Less affected by the presence of T4 autoantibodies
- Good specifcity (low false false positives)
- Low sensitivity (high false negatives)
What are the problesm of TSH test?
- High in hypothyroid dogs- can be normal
- Sometimes high with non-thyroidal illness, use of sulphonamides
Always measure in combination with tT4
What are the pros and cons of autoantibodies tests of thyroid hormones?
- Provide evidence of active inflammation/immune reaction
- Not all dogs who test positive to TGAA become clinically hypothyroid
- Cause abberent test results- falsely increased tT4,tT3
- Autoantibodies can affect results
Describe the process for diagnosing hypothyroidism
