Thyroid Endocrinology Flashcards
(86 cards)
1
Q
Obj: How are the different thyroid hormones regulated?
A
- mostly by level of Thyroid stimulating hormone from the anterior pituitary
- which is controlled by Thyrotropin releasing hormone (TRH) from the hypothalamus
2
Q
Obj: What are all the tests for thyroid function?
What do they test for?
A
- Total T4
- sensitive screening test - negative rules out hypothyroidism
- Free T4
- Screening for hypothyroidism - low fT4 strongly suggestive
- Total T3
- screening test for euthyroidism - poor
- Basal TSH conc.
- screening for hypothyroidism - need confirmation
- low cTSH is consistent with hypothyroidism if TT4/fT4 decreased
- Antithyroglobulin Antibodies
- Autoantibodies form when thyrocyte destruction exposes intracellular thyroglobulin to immune system
- Positive TgAA titer does not reflect/predict abnormal thyroid function
- Anti-T3 and Anti-T4 antibodies
- can interfere with some hormone assays
- not necessarily indicative of hypothyroidism
3
Q
What is the difference between T3 and T4?
A
- T4:
- produced exclusively in the thyroid
- T3:
- 20% from thyroid
- Most produced in peripheral tissues by enzymatic de-iodination of T4
- 3-5x mor potent than T4
4
Q
What are the affects of thyroid hormone on the Heart?
A
- Chronotropic and Inotropic effect
- Increases # of B-adrenergic receptors
- enhances responses to circulating catecholamines
- increase proportion of a-myosin heavy chain
5
Q
What are the effects of thyroid hormone on adipose tissue?
A
- Catabolic
- Stimulates lipolysis
6
Q
What are the effects of Thyroid Hormone on muscles?
A
- Catabolic
- Increase protein breakdown
7
Q
What are the effects of Thyroid Hormone on Bone?
A
- Developmental
- Promote normal growth and development
8
Q
What are the effects of Thyroid hormone on the nervous system?
A
- Developmental
- Promote normal brain development
9
Q
What are the effects of Thyroid hormone on the gut?
A
- Metabolic
- increase rate of carbohydrate absorption
10
Q
What are the effects of thyroid hormone on lipoproteins?
A
- Metabolic
- stimulate formation of LDL receptors
11
Q
What other effects do thyroid hormones have on the body?
A
- Calorigenic
- Stimulate oxygen consumption by metabolically active tissues
- except: testes uterus, lymph nodes, spleen, anterior pituitary
- Increase metabolic rate
12
Q
What is a Total T4 (TT4) test?
Pros/Cons?
A
- Common screening test
- serum sample, randomly timed
- Non-thyroidal illness and drugs can suppress TT4 ( = misdiagnosis_
- Anti-T4 autoantibodies can interfere with assay ( = Misdiagnosis)
13
Q
What is a Free T4 (FT4) test?
Pros/Cons?
A
- May be used as screening test
- Serum sample, randomly timed
- More specific than TT4 ( Fewer false positives)
- Non-thyroidal illness can suppress FT4 - but less effect than on TT4
- Anti-T4 antibodies do not interfere with assay
14
Q
What is a Basal Serum TSH level test?
Pros/Cons?
A
- Serum sample, randomly timed
- Measured by immunoassay
- Species-specific assay needed (cTSH = canine TSH)
- Poor screening test when used alone
- Cannot be used as sole diagnostic test
- More reliable when TT4 and/or FT4 levels also support the diagnosis
15
Q
How can the results of TSH and T4 tests be interpreted?
A
- TSH⇣ & T4⇡ = Hyperthyroidism
- TSH⇡ & T4⇣ = Primary Hypothyroidism
- TSH⇣ & T4⇣ = Secondary Hypothyroidism
16
Q
What are thyroid Stimulation tests?
A
17
Q
Pathways of Thyroid hormone metabolism
A
18
Q
What is Autoimmune thyroid disease?
A
- Mediated by autoantibodies against various thyroid proteins
- Follicular proteins serve as autoantigens
- Antigen-antibody complexes promote inflammation and tissue damage
19
Q
What are the anti-thyroid antibodies in dogs?
A
- Anti-thyroglobulin
- Anti-T4
- Anti-T3
20
Q
What does the presence of Anti-thyroid antibodies in dogs mean?
A
- Clinical relevance isn’t clear
- may indicate onset of autoimmunity
- Not useful for dx
21
Q
What is Hypothyroidism?
A
- Diminished production and secretion of thyroxine (T4) and triiodothyronine (T3)
22
Q
What are the different types of Hypothyroidism?
A
- Primary Disease - Thyroid is abnormal (No/Low T4 production)
- Secondary Disease - Pituitary is abnormal (No/Low TSH production)
- Tertiary Disease - Hypothalamus is abnormal (No/Low TRH production)
23
Q
Obj: what are the typical features of canine Hypothyroidism?
A
24
Q
Obj: what are the laboratory methods for Hypothyroidism diagnosis?
A
25
Obj: what are the difficulties associated with definitive diagnosis of Hypothyroidism?
26
Obj: what are the treatment options for Hypothyroidism?
27
Obj: what are the causes and features of feline hypothyroidism?
28
What are the possible Pathogenesis of Hypothyroidism?
* Lymphocytic thyroiditis
* immune-mediated pathology/genetic influence
* Presence of anti-thyroglobulin antibodies
* Idiopathic follicular atrophy
* end point of thyroiditis or distinct disorder
* Other Causes:
* Adverse drug reaction
* Neoplastic destruction of thyroid gland
* Iodine deficiency
29
What is Cretinism?
* Congenital Hypothyroidism
30
What are causes for Iatrogenic hypothyroidism?
* Thyroidectomy
* Radioiodine therapy (usually cats)
* Radiation therapy (oncologic tx)
31
What is the epidemiology of canine primary hypothyroidism?
* Most commonly **misdiagnosed** canine endocrinopathy
* Incidence rates vary from 1:150 to 1:500
* Diagnoses most frequently made in dogs at 4-10yo
32
What breeds have increased risk of primary hypothyroidism?
* Beagle
* Golden Retriever
* Great Dane
* Irish Setter
* Doberman Pincher
* OE Sheep
33
What is the common hx of dogs with Hypothyroidism?
* Depressed mentation
* Inactivity
* Cold intolerance
* Weight gain
* Infertility - females
34
What organ systems are commonly affected in hypothyroidism?
* BCS
* Skin and coat
* Ocular
* Cardiac
* Neruomuscular
35
What are the dermatologic signs of Hypothyroidism?
* Alopecia/slow hair regrowth
* Dry skin with scaling
* Dull brittle hair
* Seborrhea
* Pyoderma
* **Myxedema - thick nonpitting edema of the skin**
* pushing your finger in will not leave an indent
36
What are ocular abnormalities common with Hypothyroidism?
* Lipid accumulation - Cornea, aqueous humor, retinal vessels
37
What cardiac abnormalities are common with Hypothyroidism?
* Myocardial depression
* Sinus bradycardia
* weak apical beat
* Decreased fractional shortening
* Low voltage QRS complexes
* Sever DCM in Great Danes
38
What neuromuscular abnormalities are common with Hypothyroidism?
* Weakness (may be profound)
* Knuckling/hyporeflexia
* Slow, stiff gait
39
What reproductive abnormalities are common with hypothyroidism?
* Female infertility
40
What causes the “tragic” expression of hypothyroid patients
* Myxedema
* non-pitting edema
* Head, face, neck
41
What is a Myxedema crisis?
* Severe acute hypothyroidism
* Cerebral (CNS) signs
* Altered mentation
* Seizures, circling, head tilt
* Coma occurs infrequently
42
What CBC results are characteristic of Hypothyroidism?
* normocytic
* Normochromic
* Non-regenerative anemia
43
What abnormalities are common on the chemistry panel of hyperthyroidic patients?
* No changes are diagnostic or pathonomonic
* Lipid abnormalities:
* Hypercholesterolemia (65 - 75%)
* Hypertriglyceridemia
* Other:
* Elevated ALT, ALP
* Electrolyte disturbances
* Creatine kinase
44
How is Canine Hypothyroidism clinically diagnosed?
* Appropriate clinical signs
* Absence of other significant disease
* supportive lab results
* Screen with total T4
* Confirm with FT4 and TSH
45
What conditions/results may make hypothyroidism diagnosis difficult?
* Presence of factors that affect thyroid function:
* Breed factors - sight hounds have lower TT4
* Non-thyroidal illness ("sick-euthyroid") lowers thyroid hormone levels but not TSH
* Diabetes mellitus, HaAC, Addison's
* Inflammatory disorders, organ failure
* Sx, Starvation
* Drugs:
* corticosteroids
* Sulfa drugs - true hypothyroidism (reversible)
* Borderline or unexpected laboratory results
* low or normal TSh
* Borderline thyroid hormone levels when high likelihood Hypothyroidism
* May elect a therapeutic trial with T4 suppleentation
* need high confidence level and monitoring
46
What are the treatment options for Hypothyroidism?
* Hormone replacement:
* Synthetic L-thyroxine (T4) - preferred
* ThyroTabs is only approved veterinary formulation
* variable absorption due to poor oral availability
* BID dosing preferred for tablet preparations
47
What is the treatment for a Myxedema crisis?
* IV L-thyroxine - 5ug/kg (0.005mg/kg) IV q 12 h during crisis
48
What therapeutic monitoring should be done with hyperthyroidic patients
* Measure serum TT4 2-3 weeks after initiating treatment
* Peak serum T4 preferred - obtain serum sample 4-6hr post dosing
* Desired TT4 level: high normal or just above normal reference range
* T3 levels unreliable, no noeed to follow cTSH
49
How long does treatment response take for hypothyroidism
* Clinical response usually begins within one week
* Up to 6-8 weeks needed for full clinical response
50
What to consider when the treatment for hypothyroidism fails?
* Incorrect diagnosis
* Poor owner compliance
* Poorly controlled concurrent conditions
* Pharmacologic considerations
* Inactivated product
* Inappropriate dose
* Inappropriate frequency
* poor bioavalability
* T3 more available than T4
51
Obj: Know typical features of feline Hyperthyroidism
* Polyphagia
* Weight loss
* Hypertension
* increased defecation size and frequency
* mild erythrocytosis
* stress leukogram
* Azotemia
52
Obj: Know laboratory methods for HT diagnosis
* TT4
* T3 suppression test
53
Obj: Understand the clinical relationship between Hyperthyroidism and masked renal disease in cats
* Increase in renal blood flow secondary to hyperthyroidism increases the glomerular filtration rate and decreases serum creatinine
* Urea and creatinine values may be misleading
* affected by BCS, Thyroid hormone, CKD and hydration
* Low USG is found in HT cats w/ or w/out CKD
54
Obj: Know the advantages and disadvantages of the three major Hyperthyroidism treatment options
* Medical Management
* Pros - cost spread out, no hospital stay
* Cons - Lifelong therapy and monitoring, Adverse effects
* Radioactive Iodine
* Pros - \>95% cure rate
* Cons - patient selection crucial, must be stable, have concurrent illnesses controlled, can acclimate to hospital environment, lump sum Cost
* Dietary Therapy
* Pros: non-invasive (cats hard to medicate/hospitalize)
* Cons: Lifelong diet, SOLE diet
55
Obj: understand patient selection for each treatment option for hyperthyroidism
* Medical Management
* financial concerns
* debilitated patient
* Radioactive Iodine
* stable well established HT
* concurrent illnesses addressed
* patient can acclimate to hospital
* Dietary Therapy
* Stable enough to permit chronic dietary therapy
* concurrent illnesses addressed
* Surgery
* No ectopic thyroid tissue
* Unilateral thyroid disease
* other options limited
56
What causes Hyperthyroidim?
* Excess thyroxine (T4) and triiodothyronine (T3)
57
What kind of Endocrinopathy is Feline Hyperthyroidism?
* Primary endocrinopathy
* Arises from an autonomous change in thyroid gland
* Independent of hypothalamus (TRH) or pituitary (TSH) regulation
58
What are the different changes in the thyroid gland that result in hyperthyroidism?
* Benign Thyroid Tumors:
* multinodular adenomatous goiter (most common)
* both thyroid lobes involved
* Adenoma - single mass
* Malignant thyroid tumors
* thyroid carcinoma
59
What is the rate of the different tumors causing feline hyperthyroidism?
* Multinodular adenomatous goiter - 70-75%
* Adenoma - \<30%
* Thyroid carcinoma 1-3%
60
What is the pathogenesis of feline hyperthyroidism?
* Unknown, some theories
* Environmental influences - polybrominated diphenyl ethers (endocrine disruptors)
* lifestyle influences - indoor only
* Nutritional influences - canned food, certain flavors
61
What are the common signalment of feline hyperthyroid patients?
* Middle-age and older
* less than 5% \<10yo
* No breed predilections
* No sex predilections
62
What are the common signs of Feline hyperthyroidism?
Other signs?
* Weight loss
* polyphagia
* Other:
* unkempt hair coat
* PU/PD
* Vomiting
* Hyperactivity
* Diarrhea
* Heat intolerance
* Behavior changes
63
What are some major complications of Feline hyperthyroidsim?
* Cardiac failure/disease
* Thyroid ‘storm’ - acute release of thyroid hormones during events that illicit stress
* panting / open mouth breathing
* tachycardic
64
What does a physical exam of a cat with hyperthyroidism look like?
* Thyroid nodule (goiter) - usually present
* Abnormal BCS
* Fractious / hyperactivity / behavioral abnormalities
* Abnormal heart sounds
* tachycardia
* murmur
* Arrhythmia (gallop, VPC)
* Overgrown nails, sunken eyes
* Systolic hypertension
65
What laboratory abnormalities are common with feline hyperthyroidism?
* CBC - unremarkable / non-specific changes
* Chemistry - no pathognomonic changes
* Hyperglycemia - could be stress response
* Azotemia - variable changes in BUN and creatinine
* Symmetric dimethylarginine (SDMA) - may be elevated
* Elevated ALT /ALP, occasional mild increase in bilirubin
* ~80% of cases
* Usually NOT due to primary liver disease
* Urinalysis - non-specific changes
* specific gravity varies (1.009 - 1.055+)
66
What are the different interpretations for TT4, Free T4 and TSH tests for Hyperthyroidism
67
What is ‘Masked’ chronic kidney disease
* Abnormal kidney anatomy
* abdominal palpation / imaging findings
* Abnormal kidney function
* signs of CKD
* Urea and creatinine values may be misleading
* affected by BCS, thyroid hormone, CKD, and hydration
* Reduced urine concentration (low USG)
* in HT cats w/ or w/out CKD
* SDMA - dx usefulness unclear
* Abnormalities persist after euthyroidism restored
* changes consistent with CKD do NOT resolve or worsen after treatment for Hyperthyroidism
68
What cardiac changes can occur in feline hyperthyroidism?
* HT cats may develop a reversible hypertrophic cardiomyopathy
* all cardiac changes resolve after effective anti-thyroid treatment
* Imaging & echocardiogram are indicated when HT cat has cardiac signs
* Cardiac drugs may be needed
* cardiac conditions should be controlled before definitive treatment
69
What are the cardiac signs of Thyrotoxicosis?
* Tachycardia / Arrhythmia
* Murmur
* Respiratory Distress
* Tachypnea or panting
* Muffled heart sounds
70
What are the treatment options of Hyperthyroidism in cats?
* Medical management
* Radioactive Iodine
* best option for most patients
* Dietary Therapy
* select situations
* Surgical Thyroidectomy
* not routinely recommended
71
What is the medical management option for Feline Hyperthyroidism?
Methimazole
72
How does Methimazole work for Feline Hyperthyroidism?
* Drug concentrates in thyroid gland
* Prevents thyroid hormone production
* Initial dose is 2.5mg BID - adjust on T4 level and response
* oral and transdermal formulations
* LIFE LONG therapy and Monitoring
73
What are the possible side effects of methimazole?
* GI (inappetence, vomiting) - common
* Facial pruritis - common
* Hepatotoxicity
* Immune-mediated hemolytic anemia
* Agranulocytosis
* Thrombocytopenia
74
What occurs following methimazole withdrawal
* adverse reactions typically resolve
* Hyperthyroidism returns
75
How does Radioactive Iodine (131I) work for Hyperthyroidism
* Thyroid is the only organ that accumulates iodine
* 131I targets overactive thyroid tissue
* Emits beta radiation
* \>95% cure rate w/ single dose
76
Why would Radioactive Iodine treatment fail?
* Thyroid carcinoma 2-3%
* Low 131I dose
77
What is the ideal patient for Radioactive Iodine treatment of hyperthyroidism?
* Stable, well established Hyperthyroidism
* methimazole trial may help evaluate patient
* Concurrent illness addressed
* cardiac thyrotoxicosis, hypertension
* Patient can acclimate to hospital environment
* isolation ~7 days; hospitalized ~10 days
* state and local rules govern isolation
78
When is surgical thyroidectomy a reasonable option?
* NO ectopic thyroid tissue
* Unilateral thyroid disease
* other options limited
79
What are the advantages / disadvantages of thyroidectomy?
* Advantages:
* may be curative
* minimal oral medications
* Disadvantages:
* invasive
* Risk of anesthesia
* Hypoparathyroidism
* Hypothyroidism
80
How does a restricted iodine diet work for hyperthyroidism?
* Hill's Science Diet Y/D
* Formulation has severely restriced iodine content (≤0.3ppm)
* must be fed as sole diet
* Thyroid levels fall by 3 wks and are normal by 8 wks in most cases
* Treatment failure may occur if cat gets other food / supplements
81
What is the ideal patient for a restricted iodine diet for hyperthyroidism
* Stable enough to permit chronic dietary therapy
* Concurrent illness is addressed
82
What is the prognosis of feline hyperthyroidism?
* Curable disease
* Depends on:
* physical condition at dx
* simulatneous diseases present
* Benign vs malignant tumor
* Treatment options available
83
What is a "thyroid storm'?
* Crisis caused by acute release of thyroid hormone
* can occur as acute exacerbation of hyperthyroidism
* Induced by stress/anxiety
84
What is the goal of emergency treatment of a hyperthyroid storm? what are the options for treatment
* Immediately prevent thyroid hormone release or action
* Stable iodines:
* Potassium iodide
* Potassium iodate - 25 mg/cat q8h
* Iopanoic acid - 100mg/cat BID -
* block T4 ⇢ T3 conversion
* block T3 at receptor
* Inhibit thyroid hormone synthesis
* Beta adrenergic blockers
* Propanolol - 5 mg q8h or 0.02 mg/kg IV over 1-min
* inhibits conversion
* Atenolol - 1 mg/kg q 12-24 h
* Esmolol (B1 agent) IV 0.5 mg/kg then CRI
85
What is Iatrogenic Hypothyroidism? How can it be caught?
* Usually 131I overtreatment
* May be clinical - usually subclinical
* ~30 days post treatment - measure TT4, FT4, TSH, serum chemistry
* Overt hypothyroidism - TT4 OR FT4 is low AND TSH is high
* requires T4 supplementation
* Subclinical hypothyroidism - TT4 OR FT4 in the lower ⅓ of the reference range AND TSH is high
* cat is mildly hypothyroid
* Supplementation can be withheld unless NEW azotemia has developed
* ~3 and 6 months post treatment:
* if TT4 OR FT4 is low AND TSH is high, OR the cat has developed NEW azotemia - T4 supplementation required
* If values meet criteria for subclinical hypothyroidism - continue to monitor
86
What is a Methimazole trial?
* Purpose = evaluate effect of hyperthyroidism on renal function
* Do a trial if concerned that CKD will worsen with anti-thyroid treatment
* Protocol:
* administer methimazole until euthyroidism is restored (1-2 weeks)
* Asses renal values and clinical condition
* If no clinical signs of CKD, No indication of azotemia
* THEN - patient should be a good candidate for 131I therapy
* Successful trial does NOT predict absence of CKD with _certainty_
