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
What drugs can induce hypothyroidism?
Sulphonamides
Thyroid hormone concentrations normalise once drug therapy is withdrawn- 6 weeks
How is hypothyroidism treated in dogs?
Hypothyroid dogs require thyroid hormone replacement therapy for life- synthetic T4 is treatment of choice
* Levothyroxine
* Care in cardiac patients
* Monitor tT4- 4-6 hours post treatment
* Metabolis signs should resolve within the first few weeks
* Dermatological- 2-3 months
What are complications of hypothyroidism?
Neurological
* Peripheral neuropathy
* Generalised myopathy
* Megaoesophagus
* Laryngeal paralysis
* CNS
Hyperlipidaemia, atherosclerosis, GB, mucocoele
Myxoedema coma
What are the three different causes of hypothyroidism in cats?
What test is the most sensitive for diagnosis?
- Spontaneous- primary- rare- typical clinical signs uncommon- goiture common sign
- Secondary- due to head trauma described
- Iatrogenic- most common
Overdose of anti-thyroid admin
Bilateral thyroidectomy
Radioactive iodine
Hypophysectomy
Canine TSH most sensitive test for diagnosis
What age of cats have an increased chance of hyperthyroidism?
What cats have a decreased risk?
- Older cats- mean age 13-14, 5% over 9yo
- No breed or sex predisposition
- Decreased risk- persian, siamese, burmese, tonkinese, abyssininan, BSH
- What is the pathophyiology of most hyperthyroidism in cats?
- What may be the underlying cause?
- What are the clinical signs?
- Nodular adenomatous hyperplasia
- Nutrition (canned food), environmental, genetic
- Weight loss, PUPD, hyperactivity, GI signs, skin and hair coat changes
How is hyperthyroidism in cats diagnosed?
Haematology
* Mild to moderate erythrocytosis and macrocytosis
* increased heinz bodies
* Leukocytosis
* Increased ALP, ALT, AST, LDH
* Hypokalaemia, hyperphosphataemia
* Decreased fructosamine
How is hyperthyroidism in cats treated?
- Medical therapy
- Surgery
- Radiodine
- Others
What anti-thyroid drugs can be used to treat hyperthyroidism?
What is important for owners with use of these drugs?
Thiamazole
* Inhibits synthesis of thyroid hormone at several levels
* Check tT4 after 2 weeks then every 3 months
* Oral or transdermal
Carbimazole
* Inhibits synthesis of thyroid hormone at several levels
* Pro-drug
* Slow release form is licensed
- Owner compliance is needed- side effects occur often within the first 3 months including life threatening ones- thrombocytopenia, hepatopathy, agranulocytosis.
- If any of these reactions occur, the anti-thyroid drug must be stopped
- Trial in animals with suspected renal disease- can have normal creatinine due to muscle loss
What are the pros of thyroidectomy surgery?
What are the potential complications?
Pros
* Effective
* Available
Complications
* Iatrogenic hypoparathyroidism
* Laryngeal paralysis
* Horner’s syndrome
* Recurrent laryngeal nerve damage resulting in voice change
* Transient or permanent hypothyroidism
What is the treatment of choice for hyperthyroidism?
Radioiodine
* Brachytherapy technique
* Radioactive iodine concentrated in cells where there is upregulation of thyroid production
* Beta particles travel about 2mm in tissue
* Treats all hyperfunctioning tissue
* Curative in >95%
What is the treatment of choice for hyperthyroidism?
Radioiodine
* Brachytherapy technique
* Radioactive iodine concentrated in cells where there is upregulation of thyroid production
* Beta particles travel about 2mm in tissue
* Treats all hyperfunctioning tissue
* Curative in >95%
What other therapies can be used for feline hyperthyroidism?
Percutaneous ethanol injection
* Only solitary nodules
* Side effects due to leakage of ethanol
Hyperthermia
* Ultrasound guided heat ablation
* Transient responses
* All cases recurred within 18 months
Diet
* Iodine restricted diet
What is SHIM-RAD
Severe hyperthyroidism
Huge tumour size
Intrathoracic tumour
Multifocal disease pattern (3 or more nodules)
Resistance to Antithyroidal Drug
Some cats fail to respond to treatment- benign adenoma can transform to malignant carcinoma cells
How can CKD be affected by hyperthyroidism?
- Successful treatment of hyperthyroidism has the potential to unmask pre-existing CKD
- The associated changes in renal funciton are usually mild
- Renal function typically stabilises within 6 months of treatment
What is thyroid storm?
Thyroid storm is a condition of acute thyrotoxicosis in which the patients metabolic, thermoregultatory and cardiovasculat mechanisms are overwhelmed by excessive circulating levels of thyroid hormone
What is thyroid storm?
Thyroid storm is a condition of acute thyrotoxicosis in which the patients metabolic, thermoregultatory and cardiovasculat mechanisms are overwhelmed by excessive circulating levels of thyroid hormone
What are the most common causes of hyperthyroidism in dogs?
Functional, malignant thyroid tumour- 10-30%
Iatrogenic thyrotoxicosis secondary to oversupplementation- most common
Seen in dogs recieveing raw food diet containing thyroid tissue
