Hypothyroidism

From Dog
Symmetrical alopecia associated with hypothyroidism[1]
Distal alopecia of the tail associated with hypothyroidism[1]
Brain of a 2-year-old Australia Shepherd which died from acute neurological signs secondary to hypothyroidism, showing severe atherosclerosis of the brain and spinal cord[2]

Hypothyroidism is a neuroendocrine disease of the canine thyroid gland and characterized by thyroid inactivity or reduced activity.

This disease is far more commonly observed than hyperthyroidism in canine patients.

The disease results in reduced levels of thyroxine (T4; often around 9 pmol/L) and triiodothyronine (T3), accompanied by changes in intermediary metabolism including alterations in bodyweight, insulin resistance[3], reduced leptin and adiponectin (lipid hormone) activity[4] as well as interruption of normal blood-brain barrier function[5].

Three forms have been characterized:

  • Congenital hypothyroidism - usually result from anomalies of the thyroid gland or pituitary[6]
  • Primary hypothyroidism - adult onset lymphocytic thyroiditis or idiopathic thyroid atrophy
  • Secondary hypothyroidism - due to underlying myasthenia gravis[7][8] or deficiency of thyroid-stimulating hormone (rare)

Hypothyroidism commonly occurs in older dogs, and certain breeds are predisposed, including the Boxer (with concurrent glomerulonephritis[9]), Beagle (up to 40% affected), Tenterfield Terriers[10], Borzoi, Golden Retriever, Great Dane, Irish Setter, Doberman, Australian Shepherd Dog and Old English Sheepdog. In Golden Retrievers, progression of disease can be quite rapid.

Concurrent age-related diseases such as hypoadrenocorticism[11] and diabetes mellitus[12] often aggravate clinical symptoms[13].

Schmidt syndrome (hypoadrenocorticism, hypothyroidism and diabetes mellitus) appears relatively uncommon in dogs compared with humans[14].

Megaesophagus has been shown to not be associated with or occur as a result of hypothyroidism in dogs[15].

Clinical signs

Congenital hypothyroidism usually presents as cognitive deficits (due primarily to atherosclerosis) and skeletal developmental abnormalities, resulting in disproportionate dwarfism[16]. Goitre may or may not be present and thyroid hormones levels are usually normal[17].

In adult dogs, clinical symptoms are often mild and nonspecific and can include lethargy, weight gain, dull coat, nonpruritic, symmetrical truncal and distal tail alopecia, superficial pyoderma, infertility, cold intolerance, cardiac abnormalities, ataxia, hemiparesis, seizures, vestibular disease and polyneuropathy.[18][19]

The peripheral polyneuropathy is more commonly observed in large-breed dogs as a pelvic limb paresis, postural reaction deficits, hyporeflexia and muscle atrophy which may progress to tetraparesis over a course of 1 - 2 months[20].

Symmetrical alopecia areata, superficial pyoderma, comedones, hypertrichosis, ceruminous otitis externa and poor wound healing are also commonly observed.

In entire dogs, gynecomastia, reduced sperm motility, azoospermia is seen and in bitches, failure to conceive, prolonged interestrus length, silent estrus, anestrus and spontaneous abortions have been noted[21].

Rare symptoms such as megaesophagus[22] and xanthoma[23] and cognitive deficits have also been reported.

The subtle neurological changes seen in some dogs with hypothyroidism may be related to hyperlipidemia and hypercholesterolemia resulting in peripheral and central nerve ischemia, atherosclerosis[24] and compression by myxedematous deposits[25]

Blood analysis may be unremarkable, but alterations such as a normocytic, normochromic, nonregenerative anemia, hyperlipidemia, hypercholesterolemia, hypernatremia and hypercalcemia[26] are not uncommon. In 20% to 30% of hyperthyroid dogs, serum creatine kinase levels may be elevated due to hypothyroid myopathy or changes in muscle metabolism[27].

Diagnosis

Diagnosis is based on blood evaluation for T4 and TSH (thyroid stimulating hormone) levels, which are usually indicative.

Provocative thyroid function test and antibody ELISA testing[28] for thyroiditis may assist a diagnosis but are relatively expensive.

Technetium thyroidal uptake appears to be unreliable[29].

Thyroid biopsy and histopathology are usually required for a definitive diagnosis.

Treatment

Treatment usually involves dietary supplementation with levothyroxine (20 μg/kg every 12 hours). Clinical signs usually resolve with treatment. Failure of a response to this medication may require a re-evaluation of the underlying disease(s) process.

The use of prednisolone given concurrently for other diseases does not appear to significantly interfere with therapy[30].

The use of antioxidants (vitamins E and C, fruits and vegetables) and mitochondrial cofactors (e.g. Co-Q10, S-adenosylmethionine[31], lipoic acid and carnitine)[32] have been shown to significantly improve cognitive function in aged dogs[33] and may assist the associated polyneuropathy associated with this condition.

Additional pharmacotherapy with drugs such as selegiline (a selective irreversible MAO-inhibitor) have shown benefit in canine cognitive disorders and should be considered as an adjunct therapy in advanced clinical cases[34].

References

  1. 1.0 1.1 Animal Endocrine
  2. Blois SL et al (2008) A case of primary hypothyroidism causing central nervous system atherosclerosis in a dog. Can Vet J 49(8):789-792
  3. Hofer-Inteeworn N et al (2012) Effect of hypothyroidism on insulin sensitivity and glucose tolerance in dogs. Am J Vet Res 73(4):529-538
  4. Tvarijonaviciute A et al (2012) Effects of thyroxin therapy on different analytes related to obesity and inflammation in dogs with hypothyroidism. Vet J Sep 6
  5. Pancotto T et al (2010) Blood-brain-barrier disruption in chronic canine hypothyroidism. Vet Clin Pathol 39(4):485-493
  6. Graham PA et al (2007) Etiopathologic findings of canine hypothyroidism. Vet Clin North Am Small Anim Pract 37(4):617-631
  7. Dewey C (1995) Neuromuscular dysfunction in five dogs with acquired myasthenia gravis and hypothyroidism. Prog Vet Neurol 6(4):117-123
  8. Panciera D (1994) Hypothyroidism in dogs: 66 cases (1987-1992). J Am Vet Med Assoc 204(5):761-766
  9. Mansfield CS & Mooney CT (2006) Lymphocytic-plasmacytic thyroiditis and glomerulonephritis in a boxer. J Small Anim Pract 47(7):396-399
  10. Dodgson SE et al (2012) Congenital Hypothyroidism with Goiter in Tenterfield Terriers. J Vet Intern Med Nov 1
  11. McGonigle KM et al (2012) Mineralocorticoid Before Glucocorticoid Deficiency in a Dog with Primary Hypoadrenocorticism and Hypothyroidism. J Am Anim Hosp Assoc Oct 1
  12. Blois SL et al (2011) Multiple endocrine diseases in dogs: 35 cases (1996-2009). J Am Vet Med Assoc 238(12):1616-1621
  13. Scott-Moncrieff JC (2012) Thyroid disorders in the geriatric veterinary patient. Vet Clin North Am Small Anim Pract 42(4):707-725
  14. Adissu HA et al (2010) Lymphocytic adenohypophysitis and adrenalitis in a dog with adrenal and thyroid atrophy. Vet Pathol 47(6):1082-1085
  15. Gaynor A (1997) Risk factors for acquired megaesophagus in dogs. J Am Vet Med Assoc 211(11):1406-1412
  16. Bojanic K et al (2011) Congenital hypothyroidism of dogs and cats: a review. N Z Vet J 59(3):115-122
  17. Kuczynski LA et al (2012) Goiter with vascular anomalies in a litter of Polish Lowland sheepdogs. J Am Anim Hosp Assoc 48(4):278-283
  18. Rossmeisl JH (2010) Resistance of the peripheral nervous system to the effects of chronic canine hypothyroidism. J Vet Intern Med 24(4):875-881
  19. Tsuboi M et al (2012) Pathological Features of Polyneuropathy in Three Dogs. J Vet Med Sci Nov 1
  20. Budsburg S (1993) Thyroxine-responsive unilateral forelimb lameness and generalized neuromuscular disease in four hypothyroid dogs. J Am Vet Med Assoc 202(11):1859-1860
  21. Panciera DL et al (2012) Reproductive effects of prolonged experimentally induced hypothyroidism in bitches. J Vet Intern Med 26(2):326-333
  22. Fracassi F & Tamborini A (2011) Reversible megaoesophagus associated with primary hypothyroidism in a dog. Vet Rec 168(12):329
  23. Cramer SD et al (2011) Sellar xanthogranuloma in a dog. J Vet Diagn Invest 23(2):387-390
  24. Indrieri R et al (1987) Neuromuscular abnormalities associated with hypothyroidism and lymphocytic thyroiditis in three dogs. J Am Vet Med Assoc 190(5):544-548
  25. Jaggy A (1994) Neurological manifestations of hypothyroidism: a retrospective study of 29 dogs. J Vet Intern Med 8(5):328-336
  26. Lobetti RG (2011) Hypercalcaemia in a dog with primary hypothyroidism. J S Afr Vet Assoc 82(4):242-243
  27. Mooney C (2011) Canine hypothyroidism: a review of aetiology and diagnosis. N Z Vet J 59(3):105-114
  28. Piechotta M et al (2010) Autoantibodies against thyroid hormones and their influence on thyroxine determination with chemiluminescence immunoassay in dogs. J Vet Sci 11(3):191-196
  29. Shiel RE et al (2012) Assessment of the value of quantitative thyroid scintigraphy for determination of thyroid function in dogs. J Small Anim Pract 53(5):278-285
  30. O'Neill SH et al (2011) Effect of an anti-inflammatory dose of prednisone on thyroid hormone monitoring in hypothyroid dogs. Vet Dermatol 22(2):202-205
  31. Rème CA et al (2008) Effect of S-adenosylmethionine tablets on the reduction of age-related mental decline in dogs: a double-blinded, placebo-controlled trial. Vet Ther 9(2):69-82
  32. Head E et al (2009) Effects of age, dietary, and behavioral enrichment on brain mitochondria in a canine model of human aging. Exp Neurol 220(1):171-176
  33. Manteca X et al (2011) Nutrition and behavior in senior dogs. Top Companion Anim Med 26(1):33-36
  34. Mills D & Ledger R (2001) The effects of oral selegiline hydrochloride on learning and training in the dog: a psychobiological interpretation. Prog Neuropsychopharmacol Biol Psychiatry 25(8):1597-1613