Hypothyroidism
Introduction
- Cause: congenital or acquired metabolic condition.
- Signs: alopecia, lethargy, weight gain.
- Diagnosis: laboratory tests, response to therapy.
- Treatment: thyroxine supplementation.
- Prognosis: good but will require life-long therapy.
Presenting signs
- Lethargy, mental dullness, weight gain.
- Hair loss/thinning.
- Bilaterally symmetrical non-pruritic alopecia Skin: alopecia - overview.
- Hyperpigmentation.
- Secondary seborrhea, pyoderma Skin: deep pyoderma, malassezia dermatosis Skin: malassezia disease and demodicosis Skin: demodectic mange.
Age predisposition
- Most dogs with acquired primary hypothyroidism are middle aged.
- Acquired primary hypothyroidism is rare in dogs less than two years old.
- Dogs with lymphocytic thyroiditis tend to be younger than those with thyroidal atrophy.
- Dogs with congenital hypothyroidism usually present within the first six months of life.
Breed/Species predisposition
- Dobermann Dobermann.
- English Setter English Setter.
- Old English Sheepdog Old English Sheepdog.
- Giant Schnauzer Schnauzer: giant.
- Rhodesian Ridgeback Rhodesian Ridgeback.
- Boxer Boxer.
- Cavalier King Charles Spaniel Cavalier King Charles Spaniel.
Pathogenesis
Etiology
- In healthy animal, thyroid hormone production is under control of the hypothalamic-pituitary-thyroid axis . In most cases, disease affects the thyroid gland itself (primary hypothyroidism). Secondary hypothyroidism occurs due to diseases that reduce thyroid stimulating hormone (TSH) production by the pituitary gland. Tertiary hypothyroidism develops due to decreased thyrotropin releasing hormone (TRH) production by the hypothalamus. Due to difficulty distinguishing secondary and tertiary causes, they are often grouped as “central hypothyroidism”.
Primary hypothyroidism
- >95% of cases.
- May be acquired (common) or congenital (rare).
- The most common causes of acquired primary hypothyroidism are lymphocytic thyroiditis and idiopathic thyroid atrophy, which occur with approximately equal frequency. It is possible that lymphocytic thyroiditis may progress to idiopathic atrophy in at least some cases.
- Thyroid tumors: are rare causes of acquired primary hypothyroidism. In such cases, destruction of both thyroid lobes is necessary.
- Lymphocytic thyroiditis: characterized by diffuse infiltration of the thyroid gland by lymphocytes, plasma cells and macrophages, resulting in progressive destruction of thyroid follicles and secondary fibrosis.
- Idiopathic atrophy: is characterized by loss of thyroid parenchyma without inflammatory infiltration, and replacement by adipose tissue.
- Primary hypothyroidism can also be caused by administration of inhibitors of thyroid hormone synthesis such as thiamazole (methimazole) or potentiated sulfonamides.
- Congential primary hypothyroidism represents a group of rare disorders that cause decreased thyroid hormone production because of altered thyroid hormone synthesis or thyroid gland development.
- Iodine deficiency can cause endemic hypothyroidism, but this is extremely rare in dogs.
Secondary hypothyroidism
- <5% of cases.
- Decreased secretion of TSH from the pituitary gland leads to → decreased thyroidal follicular cell atrophy.
- Causes: pituitary tumors, hypophysectomy, congenital dysfunction of pituitary gland (secondary hypothyroidism frequently accompanies pituitary dwarfism in the German Shepherd dog German Shepherd Dog, which is caused by a mutation of the LHX3 gene).
Tertiary hypothyroidism
- Poorly defined in the dog.
- Deficient production or release of TRH, which causes reduced TSH secretion and effects on the thyroid gland as described for secondary hypothyroidism.
Pathophysiology
- In early cases, lymphocytic thyroiditis may be present without impairment of thyroid hormone secretory ability. This is known as silent thyroiditis.
- If destruction of the thyroid gland becomes more widespread, thyroid hormone concentrations tend to decrease. However, adequate thyroid hormone concentrations may be maintained by increased TSH secretion. This is known as subclinical or compensating hypothyroidism.
- When more than 75% of gland is destroyed, increased TSH secretion is no longer capable of maintaining adequate thyroid hormone secretion. Thyroid hormone concentrations decline, and clinical hypothyroidism develops. Once thyroid tissue is destroyed, the stimulus for autoimmunity is removed, leading to resolution of the lymphocytic thyroiditis in the gland and development of thyroidal atrophy. Transition from lymphocytic thyroiditis to thyroidal atrophy has been documented in a small number of dogs. It is possible that two separate pathophysiological processes may be involved in some cases.
Diagnosis
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Treatment
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Prevention
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Outcomes
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Further Reading
Publications
Refereed papers
- Recent references from PubMed and VetMedResource.
- Bennaim M, Shiel R E, Evans H & Mooney C T (2022) Free thyroxine measurement by analogue immunoassay and equilibrium dialysis in dogs with non-thyroidal illness. ResVet Sci 147, 37-43 PubMed.
- Aicher K M, Cullen J M, Seiler G S et al (2019) Investigation of adrenal and thyroid gland dysfunction in dogs with ultrasonographic diagnosis of gallbladder mucocele formation. PloS one 14 (2), 0212638 PubMed.
- Lewis V A, Morrow C M K et al (2018) A pivotal field study to support the registration of levothyroxine sodium tablets for canine hypothyroidism. JAAHA 54 (4), 201-208 PubMed.
- Shiel R E, Pinilla M, McAllister H & Mooney C T (2012) Assessment of the value of quantitative thyroid scintigraphy for determination of thyroid function in dogs. J Small Anim Pract 53 (5), 278-285 PubMed.
- Espineira M M D, Mol J A et al (2007) Assessment of thyroid function in dogs with low plasma thyroxine concentration. J Vet Int Med 21 (1), 25-32 PubMed.
- Taeymans O, Peremans K & Saunders J H (2007) Thyroid imaging in the dog: current status and future directions. J Vet Int Med 21 (4), 673-684 PubMed.
- Daminet S & Ferguson D C (2003) Influence of drugs on thyroid function in dogs. J Vet Med 17 (4), 463-472 PubMed.
- Dixon R M, Reid S W J & Mooney C T (2002) Treatment and therapeutic monitoring of canine hypothyroidism. J Small Anim Pract 43 (8), 334-340 PubMed.
- Kantrowitz L B, Peterson M E, Melián C & Nichols R (2001) Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in dogs with nonthyroidal disease. JAVMA 219 (6), 765-769 PubMed.
- Kantrowitz L B, Peterson M E, Trepanier L A, Melian C & Nichols R (1999) Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in epileptic dogs treated with anticonvulsants. JAVMA 214, 1804-1808 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.
- 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.
- Kemppainen R J, Thompson F N et al (1983) Effects of prednisone on thyroid and gonadal endocrine function in dogs. J Endocrinology 96 (2), 293 PubMed.
Other Sources Of Information
- Scott-Moncrieff J C (2015) Hypothyroidism. In: Canine and Feline Endocrinology. 4th edn. Elsevier, USA.
- Mooney C T & Shiel R E (2012) Canine hypothyroidism. In: BSAVA Manual of Canine and Feline Endocrinology. 4th edn. BSAVA, UK.