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Medicine I > Thyroid Endocrinology > Flashcards

Thyroid Endocrinology Flashcards

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
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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
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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
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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
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5
Q

What are the effects of thyroid hormone on adipose tissue?

A
  • Catabolic
  • Stimulates lipolysis
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6
Q

What are the effects of Thyroid Hormone on muscles?

A
  • Catabolic
  • Increase protein breakdown
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7
Q

What are the effects of Thyroid Hormone on Bone?

A
  • Developmental
  • Promote normal growth and development
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8
Q

What are the effects of Thyroid hormone on the nervous system?

A
  • Developmental
  • Promote normal brain development
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9
Q

What are the effects of Thyroid hormone on the gut?

A
  • Metabolic
  • increase rate of carbohydrate absorption
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10
Q

What are the effects of thyroid hormone on lipoproteins?

A
  • Metabolic
  • stimulate formation of LDL receptors
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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
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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)
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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
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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
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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
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16
Q

What are thyroid Stimulation tests?

A
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17
Q

Pathways of Thyroid hormone metabolism

A
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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
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19
Q

What are the anti-thyroid antibodies in dogs?

A
  • Anti-thyroglobulin
  • Anti-T4
  • Anti-T3
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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
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21
Q

What is Hypothyroidism?

A
  • Diminished production and secretion of thyroxine (T4) and triiodothyronine (T3)
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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)
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23
Q

Obj: what are the typical features of canine Hypothyroidism?

A
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24
Q

Obj: what are the laboratory methods for Hypothyroidism diagnosis?

A
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25
Q

Obj: what are the difficulties associated with definitive diagnosis of Hypothyroidism?

A
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26
Q

Obj: what are the treatment options for Hypothyroidism?

A
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27
Q

Obj: what are the causes and features of feline hypothyroidism?

A
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28
Q

What are the possible Pathogenesis of Hypothyroidism?

A
  • 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
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29
Q

What is Cretinism?

A
  • Congenital Hypothyroidism
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30
Q

What are causes for Iatrogenic hypothyroidism?

A
  • Thyroidectomy
  • Radioiodine therapy (usually cats)
  • Radiation therapy (oncologic tx)
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31
Q

What is the epidemiology of canine primary hypothyroidism?

A
  • Most commonly misdiagnosed canine endocrinopathy
  • Incidence rates vary from 1:150 to 1:500
  • Diagnoses most frequently made in dogs at 4-10yo
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32
Q

What breeds have increased risk of primary hypothyroidism?

A
  • Beagle
  • Golden Retriever
  • Great Dane
  • Irish Setter
  • Doberman Pincher
  • OE Sheep
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33
Q

What is the common hx of dogs with Hypothyroidism?

A
  • Depressed mentation
  • Inactivity
  • Cold intolerance
  • Weight gain
  • Infertility - females
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34
Q

What organ systems are commonly affected in hypothyroidism?

A
  • BCS
  • Skin and coat
  • Ocular
  • Cardiac
  • Neruomuscular
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35
Q

What are the dermatologic signs of Hypothyroidism?

A
  • 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
Q

What are ocular abnormalities common with Hypothyroidism?

A
  • Lipid accumulation - Cornea, aqueous humor, retinal vessels
37
Q

What cardiac abnormalities are common with Hypothyroidism?

A
  • Myocardial depression
  • Sinus bradycardia
  • weak apical beat
  • Decreased fractional shortening
  • Low voltage QRS complexes
  • Sever DCM in Great Danes
38
Q

What neuromuscular abnormalities are common with Hypothyroidism?

A
  • Weakness (may be profound)
  • Knuckling/hyporeflexia
  • Slow, stiff gait
39
Q

What reproductive abnormalities are common with hypothyroidism?

A
  • Female infertility
40
Q

What causes the “tragic” expression of hypothyroid patients

A
  • Myxedema
    • non-pitting edema
    • Head, face, neck
41
Q

What is a Myxedema crisis?

A
  • Severe acute hypothyroidism
  • Cerebral (CNS) signs
    • Altered mentation
    • Seizures, circling, head tilt
    • Coma occurs infrequently
42
Q

What CBC results are characteristic of Hypothyroidism?

A
  • normocytic
  • Normochromic
  • Non-regenerative anemia
43
Q

What abnormalities are common on the chemistry panel of hyperthyroidic patients?

A
  • No changes are diagnostic or pathonomonic
  • Lipid abnormalities:
    • Hypercholesterolemia (65 - 75%)
    • Hypertriglyceridemia
  • Other:
    • Elevated ALT, ALP
    • Electrolyte disturbances
    • Creatine kinase
44
Q

How is Canine Hypothyroidism clinically diagnosed?

A
  • Appropriate clinical signs
  • Absence of other significant disease
  • supportive lab results
  • Screen with total T4
  • Confirm with FT4 and TSH
45
Q

What conditions/results may make hypothyroidism diagnosis difficult?

A
  • 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
Q

What are the treatment options for Hypothyroidism?

A
  • 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
Q

What is the treatment for a Myxedema crisis?

A
  • IV L-thyroxine - 5ug/kg (0.005mg/kg) IV q 12 h during crisis
48
Q

What therapeutic monitoring should be done with hyperthyroidic patients

A
  • 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
Q

How long does treatment response take for hypothyroidism

A
  • Clinical response usually begins within one week
  • Up to 6-8 weeks needed for full clinical response
50
Q

What to consider when the treatment for hypothyroidism fails?

A
  • 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
Q

Obj: Know typical features of feline Hyperthyroidism

A
  • Polyphagia
  • Weight loss
  • Hypertension
  • increased defecation size and frequency
  • mild erythrocytosis
  • stress leukogram
  • Azotemia
52
Q

Obj: Know laboratory methods for HT diagnosis

A
  • TT4
  • T3 suppression test
53
Q

Obj: Understand the clinical relationship between Hyperthyroidism and masked renal disease in cats

A
  • 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
Q

Obj: Know the advantages and disadvantages of the three major Hyperthyroidism treatment options

A
  • 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
Q

Obj: understand patient selection for each treatment option for hyperthyroidism

A
  • 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
Q

What causes Hyperthyroidim?

A
  • Excess thyroxine (T4) and triiodothyronine (T3)
57
Q

What kind of Endocrinopathy is Feline Hyperthyroidism?

A
  • Primary endocrinopathy
  • Arises from an autonomous change in thyroid gland
  • Independent of hypothalamus (TRH) or pituitary (TSH) regulation
58
Q

What are the different changes in the thyroid gland that result in hyperthyroidism?

A
  • Benign Thyroid Tumors:
    • multinodular adenomatous goiter (most common)
      • both thyroid lobes involved
    • Adenoma - single mass
  • Malignant thyroid tumors
    • thyroid carcinoma
59
Q

What is the rate of the different tumors causing feline hyperthyroidism?

A
  • Multinodular adenomatous goiter - 70-75%
  • Adenoma - <30%
  • Thyroid carcinoma 1-3%
60
Q

What is the pathogenesis of feline hyperthyroidism?

A
  • Unknown, some theories
    • Environmental influences - polybrominated diphenyl ethers (endocrine disruptors)
    • lifestyle influences - indoor only
    • Nutritional influences - canned food, certain flavors
61
Q

What are the common signalment of feline hyperthyroid patients?

A
  • Middle-age and older
    • less than 5% <10yo
  • No breed predilections
  • No sex predilections
62
Q

What are the common signs of Feline hyperthyroidism?

Other signs?

A
  • Weight loss
  • polyphagia
  • Other:
    • unkempt hair coat
    • PU/PD
    • Vomiting
    • Hyperactivity
    • Diarrhea
    • Heat intolerance
    • Behavior changes
63
Q

What are some major complications of Feline hyperthyroidsim?

A
  • Cardiac failure/disease
  • Thyroid ‘storm’ - acute release of thyroid hormones during events that illicit stress
    • panting / open mouth breathing
    • tachycardic
64
Q

What does a physical exam of a cat with hyperthyroidism look like?

A
  • 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
Q

What laboratory abnormalities are common with feline hyperthyroidism?

A
  • 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
Q

What are the different interpretations for TT4, Free T4 and TSH tests for Hyperthyroidism

67
Q

What is ‘Masked’ chronic kidney disease

A
  • 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
Q

What cardiac changes can occur in feline hyperthyroidism?

A
  • 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
Q

What are the cardiac signs of Thyrotoxicosis?

A
  • Tachycardia / Arrhythmia
  • Murmur
  • Respiratory Distress
  • Tachypnea or panting
  • Muffled heart sounds
70
Q

What are the treatment options of Hyperthyroidism in cats?

A
  • Medical management
  • Radioactive Iodine
    • best option for most patients
  • Dietary Therapy
    • select situations
  • Surgical Thyroidectomy
    • not routinely recommended
71
Q

What is the medical management option for Feline Hyperthyroidism?

A

Methimazole

72
Q

How does Methimazole work for Feline Hyperthyroidism?

A
  • 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
Q

What are the possible side effects of methimazole?

A
  • GI (inappetence, vomiting) - common
  • Facial pruritis - common
  • Hepatotoxicity
  • Immune-mediated hemolytic anemia
  • Agranulocytosis
  • Thrombocytopenia
74
Q

What occurs following methimazole withdrawal

A
  • adverse reactions typically resolve
  • Hyperthyroidism returns
75
Q

How does Radioactive Iodine (131I) work for Hyperthyroidism

A
  • Thyroid is the only organ that accumulates iodine
  • 131I targets overactive thyroid tissue
    • Emits beta radiation
  • >95% cure rate w/ single dose
76
Q

Why would Radioactive Iodine treatment fail?

A
  • Thyroid carcinoma 2-3%
  • Low 131I dose
77
Q

What is the ideal patient for Radioactive Iodine treatment of hyperthyroidism?

A
  • 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
Q

When is surgical thyroidectomy a reasonable option?

A
  • NO ectopic thyroid tissue
  • Unilateral thyroid disease
  • other options limited
79
Q

What are the advantages / disadvantages of thyroidectomy?

A
  • Advantages:
    • may be curative
    • minimal oral medications
  • Disadvantages:
    • invasive
    • Risk of anesthesia
    • Hypoparathyroidism
    • Hypothyroidism
80
Q

How does a restricted iodine diet work for hyperthyroidism?

A
  • 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
Q

What is the ideal patient for a restricted iodine diet for hyperthyroidism

A
  • Stable enough to permit chronic dietary therapy
  • Concurrent illness is addressed
82
Q

What is the prognosis of feline hyperthyroidism?

A
  • Curable disease
  • Depends on:
    • physical condition at dx
    • simulatneous diseases present
    • Benign vs malignant tumor
    • Treatment options available
83
Q

What is a “thyroid storm’?

A
  • Crisis caused by acute release of thyroid hormone
  • can occur as acute exacerbation of hyperthyroidism
  • Induced by stress/anxiety
84
Q

What is the goal of emergency treatment of a hyperthyroid storm? what are the options for treatment

A
  • 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
Q

What is Iatrogenic Hypothyroidism? How can it be caught?

A
  • 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
Q

What is a Methimazole trial?

A
  • 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

Medicine I (43 decks)

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