Endocrinology - NAVDF 2021 Flashcards
(72 cards)
T/F: Most cases of canine hypothryroidism are primary hypothyroidism.
True - a result of dysfunction of the thyroid gland itself
T/F: Lymphocytic thyroiditis is a slowly progressive disease and clinical signs of hypothyroidism will not be present until at least 75% of the function of the thyroid gland is lost.
TRUE
What breeds are at increased risk for hypothyroidism based on their dog leukocyte antigen haplotype?
Doberman pinscher, Giant Schnauzer, Rhodesian ridgeback, English setter
What are possible causes of primary hypothyroidism?
lymphocytic thyroiditis, idiopathic atrophy of the thyroid gland, neoplasia, iodine deficiency, congenital hypothyroidism, radioactive iodine treatment, drugs (potentiated sulfonamides, corticosteroids, phenobarbital, some NSAIDs)
Secondary hypothyroidism results from a dysfunction of what?
pituitary gland (decreased thyrotropin/TSH production) dysfunction leading to secondary follicular thyroid atrophy
What are possible causes of secondary hypothyroidism?
pituitary tumors, congenital pituitary malformations (cystic Rathke’s pouch), pituitary trauma/surgery
Tertiary hypothyroidism results from a dysfunction of what?
hypothalamus dysfunction (decreased thyrotropin-releasing hormone)
What results in decreased TSH secretion (provides negative feedback for TSH secretion)?
thyroid hormones (thyroxine/T4; 3,5,3’-triiodothyronine/T3), somatostatin, dopamine, catecholamine, TNF-alpha, some interleukins
Why do dogs have lower serum total thyroxine/T4 levels than humans?
dogs have weaker protein binding of T4 – lower serum total T4 levels and higher free T4 levels
T/F: Thyroxine must undergo deiodination to T3 to be able to bind to the cell’s nuclear thyroid receptor.
True - although T4 has some intrinsic activity, it is often considered a prohormone as it has to be deiodinated to T3 to exert the metabolic effects of thyroid hormone
What is reverse T3?
metabolically inactive form of T3 - 3,3’,5’-triiodothyronine; this deiodination pathway increases during periods of non-thyroidal illness
Why is alopecia seen in hypothyroidism in areas of wear/tear?
thyroid hormones are very important to the skin and promote the initiation of the anagen phase of the hair follicle cycle
What dogs can have lower T4 concentrations?
older dogs and sight hounds (of any age)
Why is serum T3 not useful in making a diagnosis of primary hypothyroidism?
although T3 is the most important biologically active thyroid hormone at a cellular level, a large portion of it is produced by deiodination in peripheral tissues
T/F: Endogenous canine TSH is consistently increased in canine hypothyroidism.
False - up to 38% of hypothyroid dogs may have a cTSH concentration within the reference range; has a low sensitivity but a good specificity
What is the sensitivity and specificity of free T4 and endogenous TSH serum concentration for diagnosis of primary hypothyroidism?
sensitivity of 74-80%, specificity of 98% (overall accuracy of 86%)
Animals with lymphocytic thyroiditis can have antibodies to what components?
thyroglobulin (59% of hypothyroid dogs), T4, T3
What should be done if a dog has a positive anti-thyroglobulin antibody but normal thyroid function?
monitor for future development of hypothyroidism
What is more common: antibodies to T3 or T4?
anti-T3 antibodies
Why might an animal with true primary hypothyroidism have a normal serum T3 or T4?
anti-T3 and anti-T4 antibodies can interfere with some assays to measure T3 or T4, resulting in an erroneously high value
Glucocorticoids: effect on total T4? Free T4? Endogenous TSH?
decreased or no change; decreased or no change; no change or decreased (all changes are dose and duration-dependent)
Phenobarbital: effect on total T4? Free T4? Endogenous TSH?
decreased or no change; decreased or no change; no change or decreased
Sulfonamides: effect on total T4? Free T4? Endogenous TSH?
decreased; decreased; increased
Potassium bromide: effect on total T4? Free T4? Endogenous TSH?
no change; no change; no change
