Contributors: David Bruyette, David Scarff

 Species: Canine   |   Classification: Diseases

Introduction Pathogenesis Diagnosis Treatment Outcomes Further Reading

Introduction

  • Cause: congenital or acquired metabolic condition.
  • Signs: alopecia, lethargy, seborrhea.
  • Diagnosis: laboratory tests, response to therapy.
  • Treatment: thyroxine supplementation.
  • Prognosis: good but will require life-long therapy.
Print off the owner factsheet Hypothyroidism (Thyroid hormone deficiency) Hypothyroidism (Thyroid hormone deficiency) to give to your client.

Presenting Signs

  • Lethargy, mental dullness, weight gain.
  • Bilaterally symmetrical non-pruritic alopecia.
  • Reproductive problems.
  • Seborrhea, pyoderma, malasseziasis and demodicosis.

Age Predisposition

  • <0.5 years old (congenital).
  • 2-3 years in large and giant breeds and predisposed breeds (acquired).
  • 6-10 years in other breeds (acquired).

Breed Predisposition

Pathogenesis

Etiology

  • In normal animal, thyroid hormone production is under control of the hypothalamic-pituitary-thyroid axis Hypothalamic-pituitary-thyroid axis diagram.

Primary hypothyroidism

  • >95% of cases.
  • May be congenital (rare) or acquired.
  • Most common causes of acquired primary hypothyroidism are lymphocytic thyroiditis or idiopathic atrophy of the thyroid gland.
  • Thyroid tumors: primary or occasionally secondary, are rare causes of acquired primary hypothyroidism.
  • Lymphocytic thyroiditis: most common cause.
  • Probably auto-immune, causes diffuse infiltration of thyroid gland by lymphocytes, plasma cells and macrophages, resulting in progressive destruction of the follicles and secondary fibrosis.
  • Circulating antibodies to thyroglobulin present (found in >50% of dogs with hypothyroid dogs).
  • Idiopathic atrophy: probably a primary degenerative disorder, causes loss of thyroid parenchyma with replacement by adipose tissue.

Secondary hypothyroidism

  • <5% of cases.
  • Decreased secretion of thyroid stimulating hormone (TSH) from pituitary → secondary follicular atrophy of thyroid.
  • Cause: pituitary tumors, congenital malformation of pituitary gland in the German Shepherd dog (with pituitary dwarfism Congenital panhypopituitarism).

Tertiary hypothyroidism

  • Poorly defined in the dog.
  • Deficient production or release of TRH.

Iodine deficiency

  • Very rare.

Pathophysiology

  • Destruction of normal thyroid tissue → negative feedback increasing TSH secretion → remaining functional tissue secreting T4 and T3 at proportionately higher rates.
  • When more than 75% of gland is destroyed then T4 and T3 secretion is reduced → effects nearly every body tissue → wide range of clinical signs.

Diagnosis

Presenting Problems

  • Alopecia.
  • Reproductive cycle abnormalities.
  • Lethargy.
  • Pruritus (secondary to infection).

Client History

  • Lethargy/mental depression.
  • Increased coat shedding.
  • Weight gain.
  • Irregular estrus, anestrus, reproductive failure, gynecomastia, reduced libido.
  • Heat seeking.
  • Pruritic or seborrheic skin conditions.
  • Coat color changes.

Clinical Signs

  • In general, a combination of signs is more suggestive of hypothyroidism than any one sign.

Decreased basal metabolic rate

  • Lethargy.
  • Mental depression.
  • Hypothermia.

Skin abnormalities

  • Non-pruritic alopecia first seen around areas of friction, especially axillae, ventral neck Hypothyroidism atypical , ventral thorax, tail.
  • Later becomes bilaterally symmetrical, but tending to spare extremities.
  • Thinning of hair coat with loss of guard hairs.
  • Dull, dry, brittle, easily epilated hair coat.
  • Scaling/seborrhea Hypothyroidism Dachshund.
  • Failure of hair regrowth after clipping.
  • Hyperpigmentation Hypothyroidism hyperpigmentation.
  • Secondary pyoderma Hypothyroidism Shar Pei , malasseziasis and demodicosis.
  • Ceruminous otitis externa Hypothyroidism cerumen.
  • Myxedema (thick, puffy skin), seen as 'tragic' facial expression.
  • Comedone formation, especially ventral abdomen Comedone.
  • Lichenificaton (usually secondary to infection).
  • Hypertrichosis (usually in Setter and Retriever breeds).

Cardiovascular system

Neuromuscular and central nervous system abnormalities

  • Peripheral vestibular disease.
  • Facial nerve paralysis.
  • Myxedema coma.
  • Polyneuropathy.
  • Muscle atrophy.
  • Laryngeal paralysis Larynx: paralysis (no link has been established but seen in animals with hypothyroidism).
  • Megaesophagus Megaesophagus (no link has been established but seen in animals with hypothyroidism).
  • Encephalopathy.

Reproductive abnormalities

  • Galactorrhea.
  • Gynecomastia Gynaecomastia in male dogs.
  • Testicular atrophy.

Growth and maturation

Ocular abnormalities

Signs which may be associated with hypothyroidism

Diagnostic Investigation

  • Trial therapy should not be used to make a diagnosis of hypothyroidism:
    • Non-specific hair growth may occur in many alopecias.
    • Time taken waiting for response may delay diagnosis of other diseases.
For diagnostic plan see Hypothyroidism: diagnosis.

Biochemistry

Hematology

Hormone assay

  • Serum Total T4 (thyroxine assay Thyroxine assay (free) ):
    • Within normal range (25-40 nmol/l) - hypothyroidism unlikely.
    • Borderline low normal range (15-25 nmol/l) - not diagnostically definitive.
    • Very low with supportive signs (0-15 nmol/l) - strongly supportive of hypothyroidism.
    • T4 may be affected by non-thyroidal illness, (euthyroid sick syndrome), drug administration, protein-binding abormalities.

      60% of dogs with severe non-thyroidal illness (and 30% with moderate) may have low TT4.
  • Free T4 assay: more sensitive and specific for hypothyroidism - less affected by non-thyroidal factors than TT4.
  • Approximately 40% of dogs with severe non-thyroidal illness (and 20% with moderate non-thyroidal illness) have low FT4.
  • If measured by equilibrium dialysis unaffected by presence of auto-antibodies.
  • Serum TSH assay:
    Should be interpreted in combination with T4 assay Thyroid: T4 assay.
  • TSH assay Thyroid stimulating hormone assay.
  • TSH> normal range - suggests hypothyroidism.
  • Many hypothyroid dogs (25%) have TSH within normal range.
  • Less affected by non-thyroidal illness than T4 (increased in 8-10% dogs with non-thyroidal illness).
    Overlap between euthyroid and hypothyroid dogs.
  • TSH stimulation test:
    • Decreased or no thyroxine response to TSH TSH-TRH stimulation test graph.
    • The most accurate and reliable means of diagnosis.
    • Severe non-thyroidal disease or drugs may produce borderline response.

      Pharmacological grade TSH, no longer available and this test is not recommended with chemical grade TSH (due to risk of anaphylactic reaction).
  • TRH stimulation test Thyrotropin releasing hormone (TRH) stimulation test :
    • Decreased or no thyroxine response to thyrotropin releasing hormone (TRH) TSH-TRH stimulation test graph - supportive of hypothyroidism.
      This test is rarely used as only small response to TRH in normal dogs so difficult to interpret.
  • Serum T3 assay Thyroid: T3 assaybaseline T3 is of little value in diagnosis of hypothyroidism.

Serology

  • Antithyroid antibodies Thyroid gland: anti-T4 antibody test :
    • Rare in hypothyroid dogs.
    • Usually result in very high [tT4] >100 nmol/l.
  • Antithyroglobulin antibody test: may be high in auto-immune cases.

Histopathology

  • Skin biopsy Biopsy: skin typical of endocrine dermatopathy.
  • Hyperplastic and hyperkeratotic epidermis, follicular infundibular atrophy of deeper portion of hair follicle, most hairs in telogen. Increased melanin throughout the epidermis with hyperpigmentation. Extensive follicular plugging.
  • Thyroid gland biopsy (invasive procedure which may be non-diagnostic).
    Not to be undertaken unless all other tests prove inconclusive.

Differential Diagnosis

Treatment

Initial Symptomatic Treatment

For myxedema coma

All of Thyroxine sodium Levothyroxine (IV or orally by stomach tube).

And IV glucocorticoids.

And Antibiotics.

And Respiratory support.

And Rewarming.

Standard Treatment

Subsequent Management

Treatment

  • If serum T4 >100 nmol/l + satisfactory clinical response: decrease dose or consider once daily dosage, re-check in 4 weeks.
  • If serum total T4 = 30-100 nmol/l + satisfactory clinical response: no change necessary.
  • If serum total T4 <30 nmol/l: increase dose, recheck in 4 weeks.
  • If serum total T4 >30 nmol/l: re-evaluate diagnosis if clinical response is not satisfactory.

Monitoring

  • Monitor for signs of thyrotoxicosis (rare):
    • Restlessness.
    • Panting.
    • Polyuria/polydipsia.
    • Diarrhea.
    • Tachycardia.
    • Tachypnea.
  • Monitor at 12 weeks with serum T4 assay pre and 4-6 hours post-pill:
    • Pre-pill: minimum levels.
    • 4-6 h post-pill: peak levels.
  • Recheck serum T4 every 6-12 months and adjust dose accordingly.

Outcomes

Prognosis

  • Good: life-long therapy required.

Expected Response to Treatment

  • Increased activity and alertness during first week of treatment.
  • Improved dermatological signs, eg hair growth within 4-6 weeks characterized.
  • Full response may take 6 months.
    Hair loss may appear increased initially.

Reasons for Treatment Failure

  • Standard reasons Standard reasons for failure in a treatment.
  • Incorrect diagnosis.
  • Failure to recognize other intercurrent endocrinopathies.
  • Insufficient therapy, eg dose, frequency, product with poor bioavailability.

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Graham P (2009) Canine hypothyroidism: diagnosis and therapy. In Practice 31 (2), 77-82 VetMedResource.
  • Franch J et al (2004) Management of leishmanial osteolytic lesions in a hypothyroid dog by partial tarsal arthrodesis.Vet Rec 155 (18), 559-562 PubMed.
  • Gieger T L, Hosgood G, Taboada J et al (2000) Thyroid function and serum hepatic enzyme activity in dogs after phenobarbital administration. JVIM 14 (3), 277-281 PubMed.
  • Dixon R M & Mooney C T (1999) Evaluation of serum free thyroxine and thyrotropin concentrations in the diagnosis of canine hypothyroidism. JSAP 40 (2), 72-78 PubMed.
  • Panciera D L (1999) Is it possible to diagnose hypothyroidism? JSAP 40 (4), 152-157 PubMed.
  • Hess R S & Ward C R (1998) Diabetes mellitus, hyperadrenocorticism, and hypothyroidism in a dog. JAAHA 34 (3), 204-207 PubMed.
  • Dixon R M, Graham P A et al (1997) Comparison of endogenous serum thyrotropin (cTSH) concentrations with bovine TSH response test. Results in euthyroid and hypothyroid dogs. JVIM 11, 121.
  • Ramsey I K (1997) Diagnosing canine hypothyroidism. In Practice 19 (7), 378-383 VetMedResource.
  • Scott-Moncrieff J C & Nelson R W (1997) Response of serum canine thyrotropin (cTSH) to stimulation by thyrotropin releasing hormone (TRH) in euthyroid dogs, hypothyroid dogs and euthyroid dogs with concurrent disease. JVIM 11, 121.
  • Frank L A (1996) Comparison of TRH to TSH stimulation for evaluating thyroid function in dogs. JAAHA 32 (6), 481-487 PubMed.
  • Hall I A, Campbell K I, Chambers M D, & Davis C N (1993) Effect of trimethoprim/sulfamethoxazole on thyroid function in dogs with pyoderma. JAVMA 202 (12), 1959-1962 PubMed.
  • Ferguson D C (1994) Update on diagnosis of canine hypothyroidism. Vet Clin North Am Sm Anim Pract 24 (3), 515-539 PubMed.
  • Pancieria D L (1990) Canine hypothyroidism. Part II. Thyroid function tests and treatment. Comp Cont Ed Pract Vet 12 (6), 843-858 VetMedResource.
  • Ferguson D C (1988) The effect of nonthyroidal factors on thyroid function test in dogs. Comp Cont Ed Pract Vet 10 (12), 1365-1377 VetMedResource.
  • Kemppainen R J, Thompson F N, Lorenz M D, Munnell J F & Chakraborty P K (1983) Effects of prednisone on thyroid and gonadal endocrine function in dogs. J Endocrinol 96 (3), 293-302 PubMed.

Other sources of information

  • Gaughan K R, Bruyette D S & Jordan F R (1996) Comparison of thyroid function testing in non-greyhound pet dogs and racing greyhounds (abstract). Proceedings of 14th ACVIM forum. pp 768.
  • Nelson R W & Feldman E C (1996) Hypothyroidism. In:Canine and Feline Endocrinology and Reproduction. 2nd edn. Philadelphia: W B Saunders. pp 68-117.
  • Muller G H et al (1995) Muller and Kirk's Small Animal Dermatology. 5th edn. Philadelphia: W B Saunders. pp 691-703 (Detailed dermatology text book for in-depth reading).
  • Refsal K R & Nachreiner R F (1995)Monitoring thyroid replacement therapy. In: Current Veterinary Therapy XII. Eds: R W Kirk & J D Bonagura. Philadelphia: W B Saunders (Guidance on monitoring therapy).
  • Ferguson D C, Kemppainen R J & Beale K M (1993) Hypothyroidism. St Petersburg: Daniels Pharmaceuticals Inc (Easy reading colour booklet).
  • Kemppainen R J & MacDonald J M (1993) Canine hypothyroidism. In: Current Veterinary Dermatology - the Science and Art of Therapy. Eds: C E Griffin, K W Kwochka & J M Macdonald. 1st edn. St Louis: Mosby Year Book. pp 265-272 (Well-presented dermatology text book).
  • Peterson M E & Ferguson D C (1990) Thyroid diseases. In: Textbook of Veterinary Internal Medicine, Vol 2. Ed: S J Ettinger. Philadelphia: W B Saunders. pp 1632-1675 (Detailed coverage of physiology of the thyroid gland and diagnostic tests for hypothyroidism).

Other Sources of Information