L3: Thyroid Gland Pathogenesis and Evaluation

A set of 110 practice flashcards based on the thyroid pathogenesis and evaluation notes from the lecture video.

What are the two main types of neoplasia arising from glandular tissue?

1. adenoma

2. carcinoma

carcinomas

malignant and invasive tumor

adenoma

benign and non-invasive tumor

Which grows locally and slowly, adenomas or carcinomas?

Adenomas

which grows more rapidly, adenomas or carcinomas?

carcinomas

Which type of neoplasia commonly metastasizes, adenomas or carcinomas?

Carcinomas

which animal tends to get non-functional carcinomas and how do they appear in regards to the thyroid?

• dogs

• tend to be large and unilateral on the thyroid

which animal tends to get functional adenomas and how do they appear?

• cats

• can be palpated

• appear as a small bilateral nodule on thyroid

what does it mean if a tumor is “functional” in regards to the thyroid gland?

has the ability to manipulate thyroid gland and hormone function

In dogs cause of hypothyroidism?

Lymphocytic thyroiditis

lymphocytic thyroiditis

Immune-mediated destruction of lymphocytes in the thyroid gland

what occurs to thyroid hormones due to lymphocytic thyroiditis?

thyroid hormones will not be produce and dog will have low T4 levels

Which immune cells are involved in lymphocytic thyroiditis?

Lymphocytes, plasma cells, and macrophages.

What percentage of thyroid tissue is typically destroyed before signs appear in lymphocytic thyroiditis?

More than 60%

What is the expected time course for progression from gland destruction to clinical signs in canine lymphocytic thyroiditis?

Progression over 1–3 years with fibrosis

What is idiopathic follicular atrophy?

Gland shrinks with no inflammation

What causes secondary hypothyroidism?

Pituitary dysfunction leading to decreased TSH

Which dog breed is commonly associated with congenital hypothyroidism in notes?

German Shepherds

what is the most common type of hypothyroidism in dogs?

primary hypothyroidism accounts for 95% of cases

what type of neoplasia can cause hypothyroidism and what occurs to the thyroid?

• non-functional (carcinoma)

• gland is destroyed and hormones cannot be produced

What is the typical age range for feline hyperthyroidism?

Cats older than 9 years

What are the signs associated with excess T4 production in cats?

Hyperthyroidism with signs such as polyphagia and PU/PD

what does PU stand for?

polyuria

polyuria

excessive urination

what does PD stand for?

polydipsia

polydipsia

excessive thirst

Whatpercentage of feline thyroid adenomas are bilateral?

Approximately 75% bilateral

what is the pathology of hyperthyroidism?

1. thyroid adenoma

2. thyroid carcinoma

what does thyroid adenoma lead to in a cat with hyperthyroidism?

excessive T4 production

what does thyroid carcinoma lead to in hyperthyroidism?

rarely functional, but can metastasize

what dogs are genetically predisposed to lymphocytic tyroiditis?

1. beagles

2. doberman pinschers

what clinical signs can be seen in a cat with a thyroid adenoma?

1. polyphagia

2. PU/PD

What is non-thyroidal illness (NTI) also called?

Euthyroid sick syndrome

MCQ: A 10-year-old Lab with respiratory disease presented for evaluation of lethargy, fever, poor appetite, and mild weight loss. Thyroid panel reveal TT3 to be less than 1.0ug/dL (low). What is the most likely diagnosis?

euthyroid sick syndrome

What is the mechanism of euthyroid sick syndrome?

Protective response to reduce basal metabolic rate by suppressing thyroid hormone secretion to conserve energy.

List common causes of euthyroid sick syndrome.

1. Systemic illness

2. Medications

3. Physiological factors

what systemic illnesses can lead to euthyroid sick syndrome?

1. trauma

2. stress

3. systemic disease

4. systemic infection (respiratory infection, heart disease, and renal disease)

what medications can lead to euthyroid sick syndrome?

1. steroids

2. NSAIDS

3. antibiotics

what antibiotic can typically lead to euthyroid syndrome?

sulfur antibiotics

what physiological factors can lead to euthyroid sick syndrome?

1. age

2. hormonal cycles

what does “compensatory hypothyroidism” refer to?

euthyroid sick syndrome

What testing caveat exists for euthyroid sick syndrome?

Drug withdrawal for 6 weeks before testing to avoid misdiagnosis.

what is the most common thyroid pathology misdiagnosis in clinic?

euthyroid sick syndrome

what is the cause of euthyroid sick syndrome?

non-thyroidal illness

what is the most common cause of hyperthyroidism in cats?

thyroid adenomas

what pathology does this describe: functional tumor, excess T4 production

hyperthyroidism

what pathology does this describe: immune-mediated destruction of thyroid cells

hypothyroidism

what pathology does this describe: protective response to reduce BMR in systemic illness?

euthyroid sick syndrome

 A 10-year-old Labrador Retriever with lethargy, weight gain, and a dry coat suspected of hypothyroidism.

What is the most common cause?

lymphocytic thyroiditis (95%)

 A 10-year-old Labrador Retriever with lethargy, weight gain, and a dry coat suspected of hypothyroidism.

What tests would you run and what would you expect to see with the results?

1. TgAA: positive

2. TT4: decreased

3. TSH: increased

4. fT4: decreased

A 12-year-old cat presenting with weight loss despite a good appetite, increased thirst, and hyperactivity.

What do you suspect?

hyperthyroidism

A 12-year-old cat presenting with weight loss despite a good appetite, increased thirst, and hyperactivity.

What tests would you run and what results would you expect to see?

1. TT4: increased

2. fT4: increased

3. TSH: decreased

A dog with a history of systemic illness and medications affecting thyroid function.

What do you suspect?

hypothyroidism

A dog with a history of systemic illness and medications affecting thyroid function.

What tests would you run and what results would you expect to see?

1. TT4: decreased

2. fT4: decreased (can be low to normal)

What is the hallmark lab finding in primary hypothyroidism?

Decreased T4 (TT4)

what could cause a cat to get hypotension?

iatrogenic from medication (methimazole)

what could cause a dog to get hypertension?

iatrogenic from medication (levimazole)

what type of process is primary hypothyroidism?

SLOW

what are the names of the different thyroid tests?

1. Total T4 (TT4)

2. Free T4 (fT4)

3. TSH

4. TgAA

what does Total T4 measure?

the total amount of bound AND unbound thyroxine in the blood

what will occur with high levels of T4?

supress TSH production, maintaining a normal balance

what will occur if you have low levels of T4?

will cause pituitary gland to increase TSH levels to stimulate production of T4

what does fT4 measure?

ONLY unbound (active) thyroxine in the blood

what type of thyroxine is the active form?

unbound

what does TSH do?

regulates thyroid hormone production and aids in diagnosing thyroid disorders

what does TgAA identify?

antibodies against thyroglobulin for autoimmune thyroid disease diagnosis

antibodies to what are implicated in a TgAA test?

T3 and T4

thyroglobulin

precursor protein for thyroid hormone production

what type of tube will you use for thyroid testing?

red top (serum)

what would the expected test result be in a dog in a total T4 test?

• low TT4 would suggest hypothyroidism and can be confirmed with fT4, TgAA, and TSH

what would an TT4 greater than 2.0 indicate in a TT4 test done in a canine?

makes hypothyroidism unlikely

what is the first line test for hyperthyroidism in cats?

A total T4 (TT4) test

what result would you expect to see in a TT4 test in felines and what would this suggest?

elevated TT4 would suggest hyperthyroidism

what test in dogs more accurately reflects thyroid function?

The free T4 (FT4) test

what else should be considered in the fT4 test in dogs?

• less affected by non-thyroidal illness that TT4

• if TT4 is borderline in hypothyroidism, test can be used to confirm

what test detects early or mild hyperthyroidism in cats?

fT4 test

when can fT4 be falsely elevated in cats?

• in sick euthyroid cats

• should use TT4 as well for reliability

what will be elevated in hyperthyroid cats?

fT4

what is elevated in dogs with primary hypothyroidism (60-85% of cases)?

TSH

what is important to understand when testing TSH in canines?

• can be normal in hypothyroidism or elevated in euthyroid dogs

• cannot be used as a standalone diagnostic

what TSH level would support hyperthyroidism in cats?

undetectable TSH (less than 0.03 ng/mL)

what else can TSH help diagnose in cats?

iatrogenic hypothyroidism

what would a positive TgAA test result mean in a dog?

supports hypothyroidism, but it is not definitite diagnostic on its own

what does a TgAA test indicate in dogs?

immune-mediated lymphocytic thyroiditis

when is a low T4 level common in horses?

when horse is stressed by disease or during exercise

when should hypothyroidism NOT be diagnosed in a horse?

on a single blood sample taken from horses that are ill or training