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Endocrine > Thyroid function lab test > Flashcards

Thyroid function lab test Flashcards

1
Q

what are the thyroid hormones?

A

Thyroxine: T4, greatest amount of thyroid hormone, must be converted to “active” hormone T3

Triiodothyronine: T3, most biologically active

Calcitonin: comes from the thryoid, regulates calcium and phosphate

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2
Q

What form of T3 and T4 can penetrate cellular membranes and exert biologic activity by interacting w/ nuclear receptors?

A

-only free T3 and T4,

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3
Q

Where is T4 converted to T3?

A

especially in the liver, gut, skeletal muscle, brain, and thyroid

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4
Q

What are the thyroid function tests?

A
  • TSH*
  • Thyroxine (T4) –total serum level*
  • Triiodothyroneine (T3)– total serum level*
  • Free T4 Index- calculation of Free T4 (FTI)
  • T3 Resin Uptake (used to calculate FTI)
  • Thyroglobulin Aby (Tg-Ab)
  • Thyroid Peroxidase Aby (TPO-Ab)
  • Thyroid stimulating hormone receptor aby (anti-TSHR)
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5
Q

What is the best assessment of Thyroid function?

A

TSH–3rd generation assay is the most sensitive!

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6
Q

What disease are the following abys associated with?

  • TPO-Ab (thyroglobulin aby)
  • Tg-Ab
  • TSH receptor stimulating ab
  • TSH receptor blocking ab
A

TPO: Hashimotos

Tg-Ab: Hashimotos

TSH Stimulating: Graves

TSH Blocking: Hashimotos and sometimes Graves

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7
Q

TSH testing is used to…

A
  • dx thyroid disorder in symptomatic patient
  • screen newborns for underactive thyroid
  • monitor thyroid replacement therapy in ppl w/ hypothyroidism
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8
Q

How does TSH vary with age?

A

Higher TSH conc in older patients.

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9
Q

When might you see an elevated TSH? Low TSH?

A

Elevated TSH:

  • Hypothyroidism
  • Thyroiditis
  • Thyroid agenesis (newborns)
  • Pituitary Tumor

Low TSH:

  • hyperthyroidism
  • damate to pituitary gland that prevents it from making TSH
  • Hypothalamus insufficiency (3ry hyperthyroidism)
  • Taking too much thyroid medicine for tx of an underactive thyroid
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10
Q

What is the most prominent thyroid hormone? When is this hormone active?

A

T4, greater than 90%, nearly all of it is transported bound to proteins (TBG, albumin)

-only free(unbound) T4 is metabolically active

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11
Q

WHen would you see increased T4 levels? Decreased?

A

Increased:

  • hyperthyroidism
  • acute thyroiditis
  • pregnancy
  • increased TBG (Thyroid binding globulin)

Decreased:

  • hypothyroid states
  • pituitary insufficiency
  • hypothalamic failure
  • protein malnutrition
  • iodine insufficiency
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12
Q

How migh levels of TBG change the value of total T4?

A

Thyroxine Binding Globulin is what carries T4 in the blood, if there is excess TBG or Low levels of TBG this will cause a falsely elevated or lowered total T4.

Excess TBG=falsely elevated T4

Lower TBG=falsely lower T4

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13
Q

What does Free T4 Index measure? What does T3 resin uptake measure?

A

indirectly measures unbound T4, is a calculated product of T3 resin uptake and serum T4

T3 resin uptake measures unoccupied binding sites on TBG, NOT a measure of T3.

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14
Q

Measurement of Free T4 is indicated when…

A
  • dx hypo/hyperthyroidism
  • monitor resposne to therapy along with TSH
  • Gives quicker result to response to therapy with replacement thyroxine then TSH
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15
Q

what % of thyroid hormone is T3?

When might T3 be increased? Decreased?

A
  • less than 10% of total thyroid hormone, large proportion formed by periphreal T4-T3 in periphery (liver)
  • 70% protein bound

Increased:

  • hyperthyroidism
  • pregnancy
  • oral contraceptives

Decreased:
-androgens, phenytoin, propranolol

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16
Q

Thyroid Binding Globulin

  • what is this?
  • what happens if there is excess or deficiency of this?
  • pt with normal thyroid has unexplained high or low T4 or T3 it might be due to?
A

-protein in which most of the thyroid hormones in the blood are attached to.

  • if there is an excess or deficiency of this protein it alters the T4 or T3 measurement but does not affect the action of the hormone.
  • when TBG is elevated T3 and T4 are elevated.*

-might be d/t an increase or decrease in TBG.

17
Q

when is free thyroxine index (FTI or T7) used?

A

-evaluates thyroid function in patients with protein abnormalities (low albumin, lor or high TBG)

18
Q

What are the most common thyroid autoantibodies?

Indications for testing for these abys?

A

Most common:

  • Antithyroid Peroxidase (TPO Ab)
  • Antithyroglobulin (TgAb)
  • these abys work against thyroid peroxidase (an enzyme that converts T4-T3)

indications:

  • Hyperthyroid: hashimotos (if in hyperthyroid state), Graves Disease
  • Hypothyroid: Hashimotos, Myxedema
19
Q

Thyroid Stimulating Hormone abys

  • what are they?
  • can these cross the placenta?
  • what disease is most likely if you have TSH stimulating/blocking ab?
A
  • immunoglobulins that stimulate or inhibit thyroid hormone release
  • crosses placenta

Stimulating aby-graves diseasee (90% sensitivity)

blocking aby- hashimotos thyroiditis

20
Q

What is Thyroglobulin (Tg)?

-when might this be elevated?

A

protein precursor of thyroid hormones, may be used as a tumor marker.

  • elevated with:
  • -thyroiditis
  • -graves disease
  • -thyroid cancer
21
Q

What are the initial labs to establish hypo/hyperthyroidism?

What helps determine if true elevation or depression in T4 or T3?

A
  • inital labs: TSH and Free T4

- FTI (free thyroxine index) helps determine depression or elevation in T4 or T3

22
Q

Thyroid Cancer:

  • most common in what ages?
  • what predisposing conditions?
A
  • common in children and adults less than 30 and those greater than 60
  • thyroid cancer most common in people who have had head or neck irradiation or those with a family hx of thyroid cancer.
23
Q

IF you find a thyroid Nodule what steps will you take in dx ?

A
  • thorough hx
  • measure TSH; if low usually indicates probable over hyperthyroidism, if high more suspicious of cancer*

-THyroid US; confirms nodule presence.

24
Q

Thyroid US

  • good at indicating what?
  • characteristics of a benign nodule?
  • deciding when to do a FNA biopsy
A
  • cystic from solid nodules and accurate measurement of nodules size.
  • will not tell you if benign or malignant.

Characteristics:

  • sharp edges
  • fluid filled (cystic)
  • multi-nodular
  • no blood supply

FNA biopsy if:

  • nodule is greater than 1-1.5cm w/ suspicious US
  • risk factor noduels greater than 0.5
  • no risk factors and nodules less than 1 cm are followed with US to determine if the noduel is increasing in size.
  • serum TSH is normal or elevated
25
Q

Radionuclide Thyroid Scan

  • what is this?
  • why do this?
A

-uses radioactive or iodine tracer and special camera to measure how much tracer is being absorbed.

Why do this?

  • determine functional status of a thyroid nodule
  • measure size of goiterr prior to treatment
  • follow-up thyroid cancer pt
  • identify and determine if nodules are hot or cold
  • used to select noduels for FNA
26
Q

Interpreting a radionuclide Thyroid scan

A
  • “Hot” noduel: rapid uptake of iodine, less likely to be malignant.
  • “Cold” nodule: little or no uptake of iodine, more likely to be malignant
  • 95% of solitary thyroid nodules are benign
27
Q

FNA (Fine needle AspiratioN)

  • what does this determine?
  • results are classified as?
  • what two CA cannot be diagnosed clearly by FNA biopsy?
A

-benign vs maliganant

-benign, Follicular neoplasm, malignant, non-diagnostic
(this is not all-inclusive i just included the ends and the middle of the spectrum)

-hurthle cell and Follicular CA cannot be diagnosed by FNA bx.

28
Q

what does the result of “non-diagnostic” mean of a FNA bx?

A

-cytologically inadaquate, need core-needle bx or repeat FNA under US guidance.

Endocrine (10 decks)

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