Thyroid function and the clinical tests used to evaluate function

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  1. What is hypothyroidism
    • -A common hormone problem encountered in dogs is hypothyroidism – 1:500 dogs
    • -Thyroid gland does not secrete an adequate quantity of thyroid hormone called (levo)thyroxine
    • -Causes an overall decrease in cellular metabolic activity
    • -Many internal organs are affected, and the resulting problem depends on which organs are most affected
  2. Primary (Naturally Occuring) Causes of Hypothyroidism
    • Most common type > 95% two main causes
    • Lymphocytic Thyroiditis (50%)
    • Idiopathic Thyroid Atrophy (50%)
  3. Lymphocytic Thyroiditis
    • –Causes 50% of hypothyroidism
    • –Also known as autoimmune thyroiditis, occurs when the body makes antibodies against the thyroid gland
    • –Effectively destroys part of it, so it has less thyroxine to secrete into the bloodstream
    • –Symptoms will appear when it progresses to the point that the reserve power of the thyroid gland is affected
  4. Idiopathic Thyroid Atrophy
    • -Causes 50% of hypothyroidism
    • -Cause of thyroid gland atrophy is unknown
  5. Acquired Secondary Hypothyroidism (Euthyroid Illness)
    • -Uncommon
    • -Pituitary dysfunction or destruction - lack of thyroid-stimulating hormone (TSH)
    • Secondary to some medications or diseases
    • –Sick Euthyroid Syndrome (Euthyriod Sick Syndrome or Euthyroid Illness)
    • -Must be RULED OUT and distinguished from true thyroid illness
  6. Signs of Hypothyroidism
    • Bilaterally symmetrical truncal alopecia
    • Dull hair coat/shedding
    • Lack of hair regrowth following clipping
    • Hyperpigmentation
    • Excessive scaling
    • Recurrent skin infections
    • “Tragic” expression – nonpitting adema
    • Weight gain
    • Lethargy/inactivity
    • Mental dullness
    • Cold intolerance
    • Hypercholesterolemia (80%)
    • Normochromic normocytic nonregenerative anemia (50%)
  7. Breed disposition for hypothyroidism
    • Great Dane
    • Irish wolfhound
    • Cocker spaniel
    • Golden Retriever
    • Poodle
    • English bulldog
    • Schnauzer
    • Dachshund
    • Doberman Pinscher
    • Irish Setter
    • Old English Sheepdog
    • Miniature Schnauzer
    • Airedale terrier
    • Boxer
  8. Good 'rule out' test for Hypothyroidism
    • -T4 test - if test is normal, dog probably does not have hypothyroidism
    • -Not very specific
    • –Many false positives, further testing required if abnormal
  9. What is the first scenario if the T4 is low or low normal
    • –First scenario is called the sick euthyroid syndrome or nonthyroidal illness (NTI)
    • –Thyroid gland is normal, but there are factors that are suppressing it from secreting a normal amount of thyroxine into the bloodstream
    • –Diseases like Cushing's disease, diabetes mellitus, chronic renal failure, liver disease, and Addison's disease can cause NTI
  10. Second scnario is T4 test is low or low normal
    • -Second scenario the thyroid gland is having a problem secreting adequate thyroxine due to one of the causes previously mentioned on the primary hypothyroidism slide
    • -"True hypothyroidism"
    • -This is the hypothyroidism we need to treat with supplemental thyroxine
  11. What is true hypothyroidism
    • -thyroid gland is having a problem secreting adequate thyroxine due to one of the causes previously mentioned on the primary hypothyroidism slide 
    • -This is the hypothyroidism we need to treat with supplemental thyroxine
  12. Free T4 test by equilibrium dialysis
    • -Used to differentiate between a true hypothyroidism from Sick Euthyroid Syndrome
    • -More Specific: rules out euthyroid illness
  13. Endogenous Thyrotropin (TSH) Concentrations in Hypothyroidism
    • -INCREASED in most hypothyroid dogs  
    • -High TSH levels with low T4 or low T4 by ED strongly indicate hypothyroidism
  14. Best Case Scenario for Diagnosis of Hypothyroidism
    • Low/Borderline T4
    • Low free T4 by ED (equilibrium dialysis)
    • High TSH
    • ~+/- T3/T4 autoantibodies
    • –Not as popular anymore
    • -T4 test good for screening and monitoring response to treatment
  15. Most common endocrine disease of cats
    • -Hyperthyroidism
    • -Commonly diagnosed in late middle age to senior cats
  16. Feline Hyperthyroidism is commonly caused by
    • -hyperfunctioning benign adenoma nodules on one or both lobes of the thyroid gland that secrete T3 and T4
    • -Excess amounts of thyroid hormone are produced, which circulates through the
    • bloodstream and affects the metabolism of many internal organs
  17. 1-2% of feline hyperthyroidism is caused by
    -malignant tumor, thyroid carcinoma
  18. Signs of Hyperthyroidism
    • Weight loss
    • Polyphagia
    • Hyporexia or anorexia
    • Nervous or hyperactive
    • Dyspnea
    • Tachycadia
    • Muscle weakness
    • Vomiting (emesis)
    • Diarrhea
    • Polyuria and polydipsia
    • Poor skin and coat condition
  19. If left untreated, hyperthyroidism can cause
  20. Diagnosis of feline hyperthyroidism
    • -Clinical signs & increased T4 values
    • -up to 42% of cats that are proven to be hyperthyroid will have normal T4 values
  21. Circulating levels of thyroid hormone can fluctuate up to...
    [Diagnosis of Feline Hyperthyroidism]
    24% during any 24 hour period and up to 28% over a 2 week period
  22. Treatment of mild or early cases of feline hyperthyroidism
    Free T4 (FT4) by equilibrium dialysis
  23. Laboratory Testing in Feline Hyperthyroidism
    • Blood panel: elevations of liver enzymes, BUN, Createnine, glucose, phosphorous and bilirubin
    • dehydration from chronic V/D
    • electrolyte imbalances may exist
  24. Thyroid Gland Scintigraphy
    • -Nuclear medicine used to diagnose and determine location of abnormal thyroid tissue
    • -Allows the thyroid glands to be imaged
  25. Thyroid gland scintigraphy process
    • -Cat is injected with a very small amount of a radioactive solution technetium 
    • -This radionuclide is an iodine analog and localizes in glandular tissue (thyroid glands, salivary glands, and glands that line thestomach mucosa)
  26. Radioiodine Therapy
    • •Safest and most effective treatment
    • •Administration of radioactive iodine cures the disease
    • •Overall success rate is 96% following a single injection of iodine- 131
    • •Recurrence rate is extremely low
    • •Return to a euthyroid state is rapid - circulating thyroid hormone levels drop precipitously within 48 hours post-administration
  27. Side effects of radioiodine therapy
    No side effects but, must screen for chronic renal insufficiency prior to treatment and monitor renal enzymes afterwards
  28. The most common medication used to treat feline hyperthyroidism
    Methimazole (Tapazole)
  29. Advantage of Methimazole
    • -The drug is inexpensive
    • -This can be misleading due to the number of recheck appointments and serial thyroid
    • hormone assays needed to assure the patient is being dosed properly
  30. Disadvantage of Methimazole
    • -not a cure for the disease, but merely a treatment
    • -The drug must be administered at least once to three times a day for the remainder
    • of the cat's life
    • -Can be difficult to administer pills – can compound flavored suspensions,
    • transdermal gels 
    • -Can cause anorexia and vomiting, self induced excoriation of the face,
    • thrombocytopenia, hepatopathy
  31. Diet with hyperthyroidism
    • -Limited dietary iodine  - limits thyroid hormone production
    • -Will see improvement in about three weeks 
    • -Hill’s Y/D diet
    • -Can be difficult in multiple cat households
  32. Surgery with hyperthyroidism
    • •Surgical removal of the thyroid gland(s)
    • •Sub-capsular thyroidectomy has the advantage of being curative
    • •Low rates of recurrence
    • •Surgical risk of damaged or removal of parathyroid gland when the thyroid is removed
    • •Hypoparathyroidism – lethargy, anorexia, depression, seizures, muscle trembling, twitching, fasiculations, bradycardia
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Thyroid function and the clinical tests used to evaluate function
2014-06-12 02:21:35
cpp cal poly pomona clinical pathology ahs 305

AHS 305 Lecture 10 thyroid function Spring 2014 Dr wallace
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