05 - Hypothryoidism & Hyperthyroidism (ERIK)

Question 1

(Hypothyroidism)

(background)

1. Hypothalamus-pituitary-thyroid axis: normal thryoid function is dependent upon trophic stimulation by what?
2. TSH is produced and secreted by cell where?
3. TSH production is inhibited directly by what?
4. TSH secretion is stimulated in a tonic fashion by what?

which is produced where?

how does it get to anterior pitutiary?

Answer 1

1. thyroid stimulating hormone (TSH or thyrotropin)
2. in the anterior pituitary
3. T3 and T4 (forming a negative feedback loop)
4. thyrotropin releasing hormone (TRH) -

produced in nuerons in the paraventricular nucleus of the hypothalamus

hypophyseal portal system

Question 2

(Hypothyroidism)

(background)

(Thryoid Hormones)

1. The thyroid gland secretes two active hormonal products… what are they?
2. which is more potent?
3. catabolic or anabolic…

in physiological amounts?

in supraphysiological amounts?

Answer 2

1. T4 (thyroxine) and T3 (tri-iodothyronine)
2. T3 is 3-5 X more potent
3. anabolic

catabolic

Question 3

(2. Clinical Features of Hypothryoidism)
(a. Species/breed/sex/age affected)
1. hypothryoidism is common in dogs, rare in cats and other domestic
2. Affected dogs are usually what age?
3. large or small dogs affected more?
4. sex predilection?

Answer 3

2. 4-10
3. large
4. NO

Question 4

(2. Clinical Features of Hypothryoidism)
(b. Clinical Signs (dogs))
1. most signs associated with a reduction in basal metabolic rate…. just read them

lethargy/mental dullness (70%), alopecia/hair loss (65%), weight gain/obesity (60%), dry hair coat/excessive shedding (60%), anestrus (40% of females; male reproductive failure has also been documented), hyperpigmentation (25%), cold intolerance/hypothermia (15%), bradycardia (10%), myxedema (increased glycosaminoglycans in dermis and subcutis leading to a “tragic” facial expression with thick/drooping eyelids).

Question 5

(2. Clinical Features of Hypothryoidism)

(Clinical Laboratory Findings (dogs))

1. hyper or hypo cholesterolemia? due to what?

Answer 5

1. hyper; reduced cholesterol metabolism and clearance (80%)

(can lead to secondary lesions such as atherosclerosis and corneal lipidosis, normocytic normochromic anemia (50%), low serum T4 concentration, abnormal TSH stimulation test

Question 6

(Hypothyroidism)

(3. Common etioligies/lesions)
1. In the dog, what accounts for over 95% of cases of hypothyroidism?
2. how much of thryoid tissue must be lost before clinical signs appear?

Answer 6

1. lymphocytic thyroiditis and idiopathic thyroid atrophy (primary hypofunction)
2. 75%

Question 7

(Hypothyroidism)

(3. Common etioligies/lesions)

Lymphocytic thyroiditis is characterized histologically by multifocal to diffuse accumulations of lymphocytes, plasma cells, and macrophages in the thyroid gland interstitium. There are often lymphocytes and macrophages infiltrating follicles. Sometimes, there are lymphoid follicles in the glandular interstitium. Remaining thyroid follicles are generally smaller than normal and are lined by columnar epithelial cells. Necrotic epithelial cells may be sloughed into the follicular lumen. In the end-stage of this process there is extensive fibrosis of the thyroid with only a few scattered clusters of thyroid follicles remaining. This condition closely resembles Hashimoto’s thyroiditis of human beings, which is an autoimmune disease. An autoimmune etiology is supported by the finding of autoantibodies against thyroglobulin and other thyroid antigens in affected dogs.

Question 8

(Hypothyroidism)

(3. Common etioligies/lesions)

Idiopathic thyroid atrophy is characterized by the loss of thyroid follicles which are replaced by adipose tissue. Although inflammation is not a prominent feature of this condition, it may in fact be the end-stage of lymphocytic thyroiditis. Degenerate follicles in these thyroids are typically smaller than normal and have pale-staining or no identifiable colloid. Epithelial cells lining these follicles may be necrotic and sometimes slough into the lumen.

Question 9

(Nutritional Causes of Hypothyroidism)

1. what is goiter?
2. Dietary iodine deficiency was once a common cause of hypothyroidism in domestic animals, but due to the routine addition of iodine to animal feed, this is now uncommon in most developed nations.
3. Iodine is part of the thryoid hormones and insufficient iodine reduces ability to do what?
4. marginal iodine deficiency may be exacerbated by what?
5. Affected foals, calves, lambs and piglets have a high infant morality rate and may be born dead or weak.

They may exhibit genearlized alopecia and myxedema - which is what?

6. The thyroid gland is markedly enlarged and may impinge on what… causing what?
7. Histologically the thyroid is composed of tightly packed follicles that are lined by tall columnar epithelial cells and which contain little colloid (hyperplastic goiter).
8. If dietary thyroid is restored to these animals, the hyperplastic goiter will progress to what?

Follicular epithelial cells in colloid goiter are flattened and relatively atrophied due to what?

Answer 9

1. non-neoplastic and non-inflammatory enlargement of the thyroid glands.
2. ability of thyroid to make T4 and T3
3. presence of goitrogeinc plants (soybeans, cabbage, kale) or drugs (thiouracil and sulfonamides)
4. increased glycosaminoglycans in the dermis and subcutis
5. the larynx resulting in asphyxia
6. colloid goiter (in which thryoid follicles become markedly distended with colloid)

loss of TSH stimulation

Question 10

(Nutritional Causes of Hypothryoidism)

1. Dietary iodine overload may cause what in foals of mares fed dry seaweed that contains high levels of iodide?
2. High dietary intake of iodine (or iodide) results in inhibition of what?

which does what?

3. this mechanism is called what?

in short run is a mechanism to do what?

4. The thyroids of young animals are exposed to higher blood iodide levels than the dam due to concentration… first by what and second by what?

Answer 10

1. hyperplastic goiter and hypothyroidism (PARADOXICALLy)
2. thyroid peroxidase

decreases organification of iodine and results in decreased thryoxine formation

3. Wolff-Chaikoff effect

protect animals from massive thyroid hormone release follwing a large dietary iodine load

4. first by placenta

second by mammary gland

Question 11

(C. Congenital Dishormonogenetic goiter)

1. and inherited (autosomal recessive) condition seen in several breeds of sheep, Afrikaner cattle and Saanen dwarf goats
2. Affected animals are unable to do what?
3. A hyperplastic goiter develops in response to what?
4. Affected animals have prolonged gestation, subnormal growth rate, and abnormal wool or guard hair development

Answer 11

2. synth and secrete adequate quantities of thyroid hormones
3. continued TSH stimulation due to lack of negative feedback by T4 and T3

Question 12

(4. Skin lesions of hypothyroidism)
(a. Gross findings)
1. bilaterally symmetrical hair loss is common
2. since thryoxine stimulates the growth phase, called what? of hair follicles

hypothyroidism results in hair growth arrest in what pahse?

3. Skin often has increased scales and may be hyperpigmented

Answer 12

2. anagen

telogen (resting phase) at which point the hairs are easily shed

Question 13

(4. Skin lesions of hypothyroidism)
(b. Microscopic Findings)
1. Skin lesions of hypothyroidism fit within a pattern of skin disease often referred to as “endocrine dermatosis”. Several different endocrinologic disorders including hyperadrenocorticism, hyperestrogenism, and growth hormone deficiency (or growth hormone-responsive dermatosis) are associated with skin lesions that generally fit this same pattern and which can be difficult or impossible to differentiate solely on the basis of the skin lesions.
2. the associated lesions include what 4 things?
3. Helpful distinguishing characteristics of hypothyroidism (when present) include what?

Answer 13

2. thin epidermis

orthokeratotic hyperkeratosis

infundibular hyperkeratosis (follicular plugs)

telogen hair follicles

3. myxedema (increased glycosaminoglycans in the dermis with resultant hydration and thickening)…

and hyperpigmentation

Question 14

(B. Hyperthyroidism)

1. Background: hyperthyroidism (thyrotoxicosis) is one of the most common endocrinopathies of domestic cats. It occasionally occurs in dogs, and is rare in other domestic species. Dogs have a much more efficient enterohepatic excretory mechanism for thyroid hormones than cats (eg: glucoronic acid to conjugate phenolic compounds such as T4).

Question 15

(B. Hyperthyroidism)

(Feline Hyperthyroidism)

1. what age?
2. sex?

(clinical signs/physical exam findings)

b. Clinical signs/physical exam findings found in over 50% of affected cats include: 1) weight loss (often despite a voracious appetite), 2) hyperactivity, 3) polyphagia, 4) tachycardia, 5) PU-PD (polyuria/polydipsia), 6) heart murmur, 7) vomiting, and 8) diarrhea and/or increased fecal volume. Approximately 10-15% of cats present with signs of overt congestive heart failure (dyspnea, muffled heart sounds, and ascites). May also see weakness, fatigue, nervousness, and hyperexcitability.
c. Clinical laboratory findings: basal T4 and T3 levels are consistently higher than normal. CBC/serum biochemistry results are non-specific.

Answer 15

1. middle aged and old (m=12)
2. no predilection

Question 16

(Hyperthyroidism)

(thryoid pathology)

1. thyroid glands of hyperthryoid cats most often contain what?
2. nodular hyperplasia may involve one or both lobes of the thryoid gland.

These lesions are…

benign or malignant?

invasive?

do they metastatize?

(thyroid carcinomas are uncommon in cats)

Thyroid glands with nodular hyperplasia may be normal in size or moderately enlarged. Microscopically, foci of nodular hyperplasia are composed of follicles having irregular shapes and of widely varying sizes. Epithelial cells comprising the follicles are often irregular in size and may have enlarged nuclei and nucleoli. Follicles elsewhere in the gland are atrophied.

Thyroid lobes containing adenomas are enlarged and often may be palpated (“thyroid slip”). Microscopically, the adenomas are sharply delimited from surrounding thyroid tissue and may be surrounded by a thin capsule. Follicular structure tends to be more variable than with nodular hyperplasia and follicles are often partially collapsed and contain little colloid. Adenomas sometimes contain focal areas of necrosis, mineralization, or cystic degeneration.

(cardiac lesions)

3. what is commonly found?

Answer 16

1. discrete adenomas or one or more sites of hyperplasia or both
2. benign

no

no

3. left ventricular concentric hypertrophy with mild to moderate cardiomegaly is commonly found

in severe cases this may progress to left-sided congestive heart failure

the cardiac lesion is reversible if the hyperthryoid state is controlled

Question 17

(Canine Hyperthryoidism)

1. age?
2. breeds?
3. sex predilection?
4. Clinical findings are same as cats

(Thyroid pathology)

1. in contrast to cats, hyperthyroidism in dogs is usually associated with what?

what percentage of thyroid carcinomas are functional?

2. Are these highly malignant?

they tend to do what?

Answer 17

1. middle aged to old (7-15)
2. boxer, beagle, golden retriever
3. no
4. thyroid carcinoma

20% (so they present due to mass in neck)

2. yes

extensively invade ajacent structures such as cervical muslces, trachea, and esophagus

Invasion of veins occurs early and results in metastasis to the lung and other distant sites