Histologic and Ultrastructural Evaluation of Thyroid Lesions Associated with Hypothyroidism in Dogs

Abstract

Abstract. Thyroid lesions in 16 pet dogs with hypothyroidism were evaluated by light and electron microscopy. Lymphocytic thyroiditis, found in seven dogs, was characterized by diffuse infiltration of the thyroid gland by lymphocytes, plasma cells and macrophages with formation of some lymphoid nodules and destruction of follicles, progressing to replacement of most of the thyroid by fibrous connective tissue. The basement membrane around follicles was thick and had electron-dense deposits. The morphology of the thyroid lesions and the presence of circulating thyroglobulin autoantibodies suggested that lymphocytic thyroiditis was immune-mediated.Idiopathic follicular atrophy, found in nine dogs, was characterized by loss of thyroid parenchyma and replacement by adipose connective tissue. Degeneration of individual follicular cells was present in the early stage, with exfoliation into the colloid and interfollicular area. Most of the thyroid gland consisted of adipose connective tissue with either interspersed small follicles or individual follicular cells that had dilated rough endoplasmic reticulum, large Golgi apparatus, and intracytoplasmic microfollicles in the advanced stage. Follicular atrophy was a degenerative lesion of follicular cells of unknown cause, not associated with inflammatory destruction in the thyroid gland.Hypothyroidism in pet dogs is a metabolic disorder that can result from primary diseases of the thyroid gland or be secondary to long-standing pituitary or hypothalamic lesions that interfere with the release of thyrotropin or thyrotropin-releasing hormone. Hypothyroid dogs have a gradual decline in vigor and physical activity, and often are "heat seekers" [ 12). Varied degrees of alopecia and epidermal atrophy were reported in 48% of hypothyroid dogs [17]. Hypothyroidism in pet dogs was associated with a low level of serum triiodothyronine (38.7 k 4.1 ng/dl) and thyroxine (0.8 k 0.07 pg/dl); circulating thyroid hormone concentrations did not increase significantly after thyrotropin stimulation [7]. These changes in thyroid function were specific for hypothyroid dogs compared to dogs with other endocrinopathies or normal pet dogs. Thyroglobulin autoantibodies were found in 48% (12 of 25) pet dogs with hypothyroidism, and may be related to the cause of thyroiditis [7]. Laboratory beagles with naturally occurring lymphocytic thyroiditis also have circulating thyroid autoantibodies [ 141, but the focal thyroiditis was not associated with 299

Ultrasonographic Evaluation of the Thyroid Gland in Healthy, Hypothyroid, and Euthyroid Golden Retrievers with Nonthyroidal Illness

“…None of the healthy dogs was receiving any medication except routine heartworm preventive and flea control at the time of the study and none had a history of illness or treatment with glucocorticoids for 6 months before entry into the study. Abnormalities identified on physical examination included mild systolic heart murmur (2 dogs), mild bilateral conjunctivitis (1), and obesity (1). Results of CBC and serum biochemistry were within reference ranges for each dog.…”

“…The mean age of the hypothyroid dogs was 8.6 years (range, 5.0-12.9 years); 5 dogs were female or spayed female and 6 were male or castrated male; and mean body weight was 35 kg (range, 25-50 kg). Clinical signs and abnormalities identified on physical examination included obesity (5 dogs), lethargy (2), weakness (2), hyperpigmentation (2), sparse haircoat (2), dermatitis (2), bradycardia (2), otitis externa (1), cricopharyngeal dysmotility (1), and head tilt (1). Abnormalities identified on CBC and serum biochemistry included mild normocytic, normochromic, nonregenerative anemia (PCV, 39%) in 2 dogs, hypercholesterolemia (mean, 490 mg/dL; range, 369-659 mg/ dL; reference range, 135-345 mg/dL) in 8 dogs, and increased serum alkaline phosphatase activity (175 and 258 U/L; reference range, 15-127 U/L) in 2 dogs.…”

“…The mean age of the euthyroid dogs with NTI was 6.2 years (range, 0.9-16.0 years); 16 dogs were female or spayed female and 19 were male or castrated male; and mean body weight was 34 kg (range, 22-48 kg). Clinical signs and abnormalities identified on physical examination included obesity (10 dogs), weakness (8), dermatitis (8), chronic otitis externa (8), lethargy (4), endocrine alopecia (3), stridor (3), hyperpigmentation (2), regurgitation (2), Horner's syndrome (2), seborrhea (1), and change in bark (1). Disorders included obesity (10 dogs), allergic dermatitis (flea, inhalant, or food; 8), weakness caused by neuropathy (8), otitis externa (8), cricopharyngeal dysmotility (4), laryngeal paralysis (3), and Horner's syndrome (2).…”

“…Destruction of the thyroid gland usually is a result of immune-mediated lymphocytic thyroiditis or idiopathic thyroid gland atrophy. [1][2][3] The resultant deficiency in circulating thyroid hormone concentrations affects the metabolic function of almost all organ systems, resulting in a myriad of potential clinical signs. 4 The diagnosis of hypothyroidism is based on clinical signs, findings on physical examination, results of CBC, serum biochemistry and tests of thyroid gland function, and response to thyroid hormone replacement therapy.…”

Thyroid Sonography as an Effective Tool to Discriminate between Euthyroid Sick and Hypothyroid Dogs

“…9,10 Most canine hypothyroidism results from an atrophy of the thyroid gland caused by lymphocytic thyroiditis or idiopathic follicular atrophy. 11,12 No diagnostic method yet has been established to measure these morphological changes in canine hypothyroidism. In contrast, in humans thyroid sonography routinely is used for the detection of morphological changes in the thyroid gland.…”