Contributors: David Bruyette, David Scarff
Species: Canine | Classification: Diseases
- Cause: congenital or acquired metabolic condition.
- Signs: alopecia, lethargy, seborrhea.
- Diagnosis: laboratory tests, response to therapy (see Hypothyroidism diagnostic flowchart).
- Treatment: thyroxine supplementation.
- Prognosis: good but will require life-long therapy.
- Lethargy, mental dullness, weight gain.
- Bilaterally symmetrical non-pruritic alopecia.
- Reproductive problems.
- Seborrhea, pyoderma, malasseziasis and demodicosis.
- <0.5 years old (congenital).
- 2-3 years in large and giant breeds and predisposed breeds (acquired).
- 6-10 years in other breeds (acquired).
- In normal animal, thyroid hormone production is under control of the hypothalamic-pituitary-thyroid axis .
- >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.
- <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 ).
- Poorly defined in the dog.
- Deficient production or release of TRH.
- Very rare.
- 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.
- Reproductive cycle abnormalities.
- Pruritus (secondary to infection).
- 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.
- In general, a combination of signs is more suggestive of hypothyroidism than any one sign.
Decreased basal metabolic rate
- Mental depression.
- Non-pruritic alopecia first seen around areas of friction, especially axillae, ventral neck , 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 .
- Failure of hair regrowth after clipping.
- Hyperpigmentation .
- Secondary pyoderma , malasseziasis and demodicosis.
- Ceruminous otitis externa .
- Myxedema (thick, puffy skin), seen as 'tragic' facial expression.
- Comedone formation, especially ventral abdomen .
- Lichenificaton (usually secondary to infection).
- Hypertrichosis (usually in Setter and Retriever breeds).
- Bradycardia Heart: dysrhythmia.
- First degree heart block Heart: first degree atrioventricular block.
- Reduced left ventricular function.
Neuromuscular and central nervous system abnormalities
- Peripheral vestibular disease.
- Facial nerve paralysis.
- Myxedema coma.
- 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).
Growth and maturation
- Dwarfism Congenital panhypopituitarism , and impaired mental development (cretinism), in congenital and juvenile onset hypothyroidism.
- Corneal lipidosis Cornea: lipidosis.
- Corneal ulceration Cornea: spontaneous chronic corneal epithelial defects (SCCEDs).
- Keratoconjunctivitis sicca Keratoconjunctivitis sicca.
- Anterior uveitis Uveitis.
Signs which may be associated with hypothyroidism
- 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 diagnostic flow chart.
- Hypercholesterolemia Blood biochemistry: cholesterol.
- Hypertriglyceridemia Blood biochemistry: triglycerides.
- Increased serum creatine phosphokinase Blood biochemistry: creatine phosphokinase.
- Increased serum lactate dehydrogenase Blood biochemistry: lactate dehydrogenase , alanine transaminase Blood biochemistry: alanine aminotransferase (SGPT, ALT).
- Normocytic, normochromic, non-regenerative anemia Hematology: hemoglobin concentration (often mild).
- Platelet Hematology: platelet count numbers and size may be abnormal.
- 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 .
- 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 .
- 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 :
- Serum T3 assay:baseline T3 is of little value in diagnosis of hypothyroidism.
- 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.
- 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.
- Other causes of alopecia Skin: alopecia - overview :
- Other causes of bradycardia:
Initial Symptomatic Treatment
For myxedema coma
All of Thyroxine sodium Levothyroxine (IV or orally by stomach tube).
And IV glucocorticoids.
And Respiratory support.
- Thyroxine sodium Levothyroxine (10-20 mcg/kg BID PO).
- Gradually if cardiac disease Heart: congestive heart failure , diabetes mellitus Diabetes mellitus , hypoadrenocorticism Hypoadrenocorticism , liver disease, respiratory failure Kidney: chronic kidney disease (CKD) (2.5 mcg/kg increasing by 2.5 mcg/kg each week until full therapeutic dose reached).
- 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.
- Monitor for signs of thyrotoxicosis (rare):
- 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.
- 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.
- 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).