feline hyperthyroidism

• Hyperthyroidism is not common in dogs but is linked nutritionally to raw food diets
• The cause is unknown and could be either due to…

  • Environmental factors
  • Thyroid disruptors? (fire retardants)
  • Nutritional factors
  • Iodine deficiency?
  • Thyroid disruptors in food? (Bisphenol-A –can lining)
• They can get a hypertrophic cardiomyopathy secondary to hyperthyroidism and hence these cases may present as a cardiac case initially
• typical clinical signs

  • Weight loss / Muscle wasting / Thin body condition
  • Increased appetite
  • Vomiting
  • Hyperactivity
  • PU/PD
  • Diarrhoea/increased faecal volume
  • Goitre (occurs in 98% of cats)
  • Be sure to palpate the entire neck thoroughly as the bigger the mass the lower the goitre can fall (with some entering the thoracic cavity)
  • Tachycardia greater than 240 bpm
  • Cardiac murmur
  • Hypertension
  • Aggressiveness, reduced tolerance to stress
• Specific tests include….

  • Total T4 (increased in 90-95% of hyperthyroid cats, day to day fluctuations, suppresses by non-thyroidal disease, immunoassays may underestimate the level)
  • Free T4 (unbound, can diffuse into the cell, more sensitive (increases chances of false positives), not affected by non-thyroidal illness)
  • T3 (active form of thyroid hormone, derived from T4 in extrathyroidal tissues)
  • TSH (canine) –non-thyroidal illness affects it.
  • Scintigraphy
• what is rT3 (reverse T3?

  r3 is an inactive form of T3 that is produced in the body (as a by product of thyroid hormone metabolism), particularly during periods of stress, and is elevated in sick euthyroid syndrome but otherwise has no clinical value. It is thought that this weakly binds to T3 diverting T4 away from T3 production
• Bland urine sediment = a sample that shows very few or no abnormal elements.
• Active urine sediment = a sample with an increased presence of abnormal elements such as RBC, WBC, bacteria, crystals or other particles
• euthyroid sick syndrome = non-thyroidal disease affecting thyroid hormone levels.
• isosthenuria = The excretion of urine that is not more concentrated or less dilute than the protein-free plasma

  • So is 1.008 - 1.012
• hyposthenuria = The secretion of urine of low specific gravity (dilute) due to the inability of the kidney to concentrate the urine normally

  • Urine more dilute than plasma
  • Lower than 1.008
• hypersthenuria = The secretion of urine of high specific gravity (concentrated)
• Total T4 is the best initial test, appropriate clinical signs plus high TT4 is very indicative of hyperthyroidism

  • High TT4 with a few signs only = retest
  • Normal TT4 and supportive clinical signs = doesn’t rule out hyperthyroidism
• The treatment options include…

  • Radioactive Iodine
  • Surgery
  • Anti-thyroid medication
  • Dietary management
• Sarcopenia = loss of skeletal muscle mass
• • Some recommend an ultra-low iodine diet. The concept is that no iodine means no thyroid hormone production. This manages but does not cure the disease or reverses the adenomatous changes in the thyroid.
  • However this diet must be fed exclusively and has a questionable palatability with a low protein content
• Scintigraphy is not routinely done in the UK but is done after an injection because it…

  • Confirms hyperthyroid diagnosis
  • Excludes diagnosis in euthyroid cats with falsely high thyroid concentration (free T4 can be elevated in 10-30% of euthyroid cats due to non-thyroidal illness therefore high free T4 does not confirm hyperthyroidism on its own)
  • Aids diagnosis of carcinoma
• Radioactive Iodine treatment requires the administration of 131I (radioactive iodine) via a subcutaneous injection. It is simple and effective but requires licensing and hospitalization facilities as the cats become ‘radioactive’ so have to be isolated (also all urine and faeces need to be disposed of correctly as they will also be radioactive)

  • This is curative but targets euthyroid not hypothyroid
• why do we care about parathyroid glands?

  they maintain the blood calcium concentration (in the bones and blood) so removal could induce hypocalcaemia
• how would you manage the removal of the parathyroid glands?

  initially corrected through parenteral and oral calcium supplementation and then vitamin D supplementation is used for long term balance. Feed diets high in calcium and low in phosphorous that are supplemented with calcium (gluconate or lactate and vitamin D3
• Hyperthyroid cats are often hypertensive so monitoring BP is important to ensure TOD doesn’t occur
• Secondary dermatitis can occur with hyperthyroid treatments