Endocrinology > Thyroid labs and imaging > Flashcards

Thyroid labs and imaging Flashcards

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1
Q

Thyroxine (T4)

A

greatest amount of thyroid hormone

- T4 must be converted to the active hormone T3

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2
Q

Triiodothyronine (T3)

A
  • most biologically active
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3
Q

Calcitonin

A

produced in parafollicular cells (C cells) of the thyroid

  • involved in regulating blood levels of Ca and phosphate by inhibiting osteoclast activity
  • decrease resorption of Ca in the kidneys
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4
Q

Thyroid hormone metabolism

A
  • thyroid hormones are poorly soluble in the plasma
  • 0.03% of T4 and 0.3% of T3 unbound
  • only free T3 and T4 can penetrate cellular membranes and exert biologic activity by interacting w/ nuclear receptors
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5
Q

Where is T4 converted to T3 at?

A
  • *liver, gut, skeletal muscle, brain and thyroid
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6
Q

3 makor plasma proteins responsible for transport?

A
  • TBG- thyroxine binding globulin
  • TBPA- thyroxine binding pre-albumin
  • albumin
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7
Q

Thyroid fxn tests?

A
  • TSH
  • T4 - total serum level
  • T3 - total serum level
  • Free T4 index - calcultaion of Free T4 (FTI)
  • T3 resin uptake (used to calc. FTI)
  • thyroglobulin AB (Tg-Ab)
  • thyroid peroxidase AB (TPO-Ab)
  • TSH receptor ab (Anti-TSHR)
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8
Q

What is the best assessment of thyroid fxn?

A
  • TSH
  • assuming steady state conditions and absence of pituitary or hypothalamic disease
  • 3rd generation assay is the most sensitive
  • direct measurements of serum thyroid hormone levels still impt in some pts
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9
Q

Thyroid ab tests?

A
  • TPO-Ab and Tg-Ab (high concentrations are seen in nearly all pts with Hashimotos thyroiditis)
  • TSH receptor stim. AB seen in graves
  • TSH receptor blocking Ab seen in atrophic Hashimoto’s thyroiditis and sometimes graves
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10
Q

mechanism of TSH?

A
  • secreted by the pituitary: responds to low levels of thyroid hormones, and responds to TRH (comes from hypothalamus)
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11
Q

TSH testing is used for what?

A
  • dx a thyroid disorder in a person with sxs
  • screen newborns for an underactive thyroid
  • monitor thyroid replacement therapy in people with hypothyroidism
  • dx and monitor female infertility problems
  • help evaluate fxn of pituitary gland
  • screen adults for thyroid disorders as recommended by American thyroid association
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12
Q

interpretation of TSH normals>

A
  • differ b/t adults, newborns and cord

- will differ from lab to lab

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13
Q

What happens to TSH as you age?

A
  • age related shift towards higher TSH concentrations in older pts
  • values will vary depending on lab
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14
Q

When will you see an elevated TSH?

A
  • hypothyroidism
  • thyroiditis
  • thyroid agenesis (newborns)
  • pituitary tumor
  • other severe and chronic illnesses
  • drug effects: iodine, ad thyroxine (T4)
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15
Q

When will you see a low TSH?

A
  • hyperthyroidism
  • damage to pituitary gland that prevents it from making TSH (secondary hypothyroidism)
  • hypothalamus insufficiency (tertiary hyperthyroidism)
  • taking too much thyroid med for tx of underactive thyroid gland
  • drugs: excess T4 therapy, glucocorticoids, L -drops
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16
Q

Importance of T4 (thyroxine)

A
  • prominent thyroid hormone (>90%)
  • nearly all of it is transported bound to proteins: TBG, albumin, transthyretin (TBPA)
  • only free (unbound) T4 is metabolically active
  • total T4 measures bound and free hormone
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17
Q

Total T4 reliable test except for what interfering factors? - this measure both bound and free T4

A
  • decreased by PTU, NSAIDs, androgens, lithium, phenytoin, amidarone, salicylates, corticosteroids, and rifampin
  • increased by estrogens, heroin, amphetamines, OCP, pregnancy ( due to increased circulating protein)
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18
Q

When will you see increased Total T4 levels?

A
  • hyperthyroidism
  • acute thyroiditis
  • conditions causing increased TBG (thyroid binding globulin)
  • pregnancy
  • meds: estrogens, heroin,, amphetamines, OCPs
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19
Q

When will you see decreased total T4 levels?

A
  • hypothyroid states
  • pituitary insufficiency
  • hypothalamic failure
  • protein malnutrition/depletion
  • iodine insufficiency
  • numerous other non-thyroid illnesses (CRF, cushings, cirrhosis, advanced cancer)
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20
Q

Why will altered levels of TBG change the value of the total T4?

A
  • direct measurement of thyroxine binding globulin (TBG) can be done and will explain the abnorm. value
  • excess TBG or low levels of TBG are found in some families as a hereditary trait. It causes no problem other than falsely elevating or lowering the total T4 level
  • these people are frequently misdx as being hyperthyroid or hypothyroid but they don’t have a thyroid problem and they don’t need any tx
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21
Q

What does the free T4 index measure?

A

indirectly measures unbound T4

  • correction of misleading results of total T4 caused by conditions that alter the TBG
  • calculated product of the T3 resin uptake and serum T4
  • T3 resin uptake measures unoccupied binding sites on TBG, it isn’t a measure of T3
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22
Q

What is a more accurate test to measure Free T4?

A
  • Free T4
    fewer interfering factors:
    increased by heparin, ASA, and propranolol
    decreased by: furosemide, phenytoins
  • various wats to test free T4 but none of them directly measures unbound T4
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23
Q

Indication for measuring free T4?

A
  • along with TSH, to dx hypo/hyperthyroidism
  • monitoring response to therapy along with TSH
  • gives a quicker result to response to tehrapy with replacent thyroxine than TSH
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24
Q

Triiodothyronine (Total T3)

A
  • accounts for less than 10% of total thyroid hormone, large proporton formed by peripheral T4 to T3 conversion (liver)
  • 70% protein bound
  • less accurate test
  • interpretation: increased in hyperthyroidism, increased during pregnancy and by OCPs, and estrogens

decreased by androgens, phenytoin, propranolol, high dose salicylates

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25
How do you calculate Free T4 using T3 resin uptake test?
- incubate pt's serum w/ radiolabeled T3 tracer - then add an insoluble resin that traps remaining unbound radiolabeled T3 - the value reported is the % tracer bound to the resin - the number of free binding sites is determined by both binding protein levels and endogenous hormone production - thyroid hormone binding ratio (THBI) = pts T3 resin/normal pool resin - T3 resin uptake measures unoccupied binding site on TBG, it isn't a measure of T3
26
T3 resin uptake: in hyperthyroidism would be?
- high T4, high T3 resin uptake or THBI, high free index T4
27
TBG excess would have what T3 resin and T4 levels?
- high T4 - low T3 resin uptake or THBI - normal free index T4
28
Hypothyroidism levels of T3 resin and T4?
- low T4, low T3 resin uptake and low free index T4
29
TBG deficiency levels of T3 resin and T4?
- low T4, high T3 resin, normal free index of T4
30
What is the Thyroid binding globulin?
- most of thyroid hormones in blood are attached to protein called TBG - if there is an excess or deficiency of this protein it alters the T4 or T3 measurement but doesn't affect the action of the hormone - if the pt appears to have normal thyroid fxn but an unexplained high or low T4 or T3 it may be due to an increase or decrease of TBG - direct measure can be doen and will explain abnormal value - excess or low TBG are found in some families as hereditary trait, it causes no problem except falsely elevateing or lowering T4 level - these pts are frequently misdx as being hyperthyroid or hypothyroid but have not problem
31
TBG levels
- major thyroid hormone transport protein - when TBG is elevate, T3 and T4 are elevated - affects the measurement of total T4/T3
32
TBG levels are increased by?
- pregnancy - infectious hepatitis - estrogens (OC, tamoxifen)
33
TBG levels are decreased by?
- protein losing conditions - malnutrition - major stress (steroids included) - androgens (testosterone) - drugs: phenytoin, propanolol - menopause
34
What does the Free Thyroxine index (FTI or T7) measure?
- evaluates thyroid fxn in pts with protein abnormalities (low albumin, low or high TBG) - calculated value, corrects for changes in protein binding, more closely approximates true hormone level
35
Most common thyroid autoantibodies?
- antithyroid peroxidase (TPO Ab) - antithyroglobulin (TgAb) - these abs work against thyroid peroxidase, an enzyme that plays a part in T4 to T3 conversion and synthesis process - causes chronic inflammation and destruction resulting in chronic thyroiditis - initially produces a mild hyperthyroidism: eventually leads to hypothyroid condition
36
Indications for testing for thyroid autoabs?
- hyperthyroid conditions: Hashimotos thyroiditis (if in hyperthyroid state), Graves disease - hypothyroid conditions: Hashimoto's thyroiditis, myxedema
37
Interpretation of thyroid autoantibodies?
- nomral healthy people produce abs especially elderly women | - normal titers:
38
TSHR antibodies?
- immunoglobulins that stimulate or inhibit thyroid hormone release - autoimmune process and this can cross the placenta: neonatal hyperthyroidism (especially in neonate whose mother has Graves)
39
Testing results of TSHR - stim ab
- graves disease (sensitivity 90%) - TSHR stim Ab decline in graves tx with antihyperthyroid meds - TSHR stim ab will initially rise after radioiodine tx and then gradually drop although sometimes they may persist for years - TSHR stim. Ab will completely go away after a thyroidectomy - * used to monitor level of disease and monitor progress of meds - TSHR blocking ab some pts with hashimoto's thyroiditis have these - leads to hypothyroidism
40
What is thyroglobulin?
- protein precursor of thyroid hormones | - levels are low or undetectable with normal thyroid function
41
When would thyroglobuin levels be elevated?
- thyroiditis - graves disease - thyroid cancer
42
When is thyroglobulin used as a tumor marker of thyroid tissue?
- effectiveness of cancer tx - residual tissue post-op - metastasis
43
When are TSH and free T4 tests done?
- initial labs to establish hypothyroidism or hyperthyroidism, also used to monitor therapy
44
WHen do you do a FTI (free thyroixine index)??
- this can help determine if true elevatiin or depression of T4 or T3
45
What does a TBG (thyroxine binding globulin) measure?
- amount of binding proteins
46
When would you test anti Tg and anti TPO?
- when you are looking for AI diseases such as Hashimotos or graves
47
When would you do a a TSH stim Ab/ TSH-blocking Ab test?
- in graves disease and hashimotos
48
What is a thyroglobulin test used for?
tumor marker
49
When are thyroid nodules brought to attention?
- when noted by pt - found by provider on PE - incidentalomas - found during other radiologic procedure (carotid US, neck CT)
50
Who presents with thyroid cancer the most?
- present in 4-6.5% of nodules - there is increased prevalence in children, adults 60 yo - people who have had head or neck irradiation, people with a family hx of thyroid cancer
51
Steps to dx thyroid nodules?
- first obtaina throrough history and PE - next measure TSH: if low usually indicates overtt hyperthyroidism - if high: more suspicious for cancer - next step: Thyroid US - to confirm presence of nodularity, assess sonographic features of nodule, assess for add. nodules and lymphadenopathy, assess for nodules for suspicious findings
52
Indications for ultrasound?
- US good at diff. cystic from solid nodules but won't be able to tell if nodlue benign or malignant, can aid in determining which nodule to bx - allows accurate measurement of nodule;s size and can determine if a nodule is getting smaller or is growing larger - aids in performing thyroid FNA bx by improving accuracy if nodule can't be felt easily on exam
53
What US characteristics suggest a benign nodule?
- nice sharp edges around nodule - fluid filled (Cystic) - not live tissue - multiple nodules (multi-nodular goiter) - no blood supply - not live tissue
54
When should a FNA bx be done?
- most accurate method for evaluating thyroid nodules and selecting pts for surgery - FNA bx of nodules >1-1.5 cm w/ suspicious findings on US - if there are any RFs nodules =/>0.5 cm are bx - pts without RFs nodules
55
What levels of TSH are an indication that a FNA bx is needed?
- when serum TSH is normal or elevated
56
What should be done if TSH is low?
- this can indicate that there is overt or subclinical hyperthyroidism - possibility that the nodule is hyperfunctioning, radioactive thyroid scan needs to be done next - don't do a FNA, not as suspect
57
Using a radionuclide thyroid scan?
- uses either a radioactive or iodine tracer and special scanner to measure how much tracer is being absorbed: - iodine: shouldnt be done on pregnant women because it can cause fetal complications - technetium-99m (isotope uptake scan) - some cancers will look hot or warm on technetium scans but cold on iodine scans so it is usually better to do on iodine scans
58
Process of radinuclide thyroid scan?
- painless procedure - takes about 30 minutes - exposes pt to little or no radiation - somewhat costly
59
Why would you do a radionuclide thyroid scan?
- to determine functional status of a thyroid nodule - measure size of goiter prior to tx - follow up thyroid cancer pts after surgery - ID nodules and determining if hot or cold - locating thyroid tissue outside of neck, ex: base of tongue or in chest - used to select nodules for FNA
60
How do you interpret a radionuclide thyroid scan?
- gland may concentrate iodine normally but will be unable to convert the iodine to thyroid hormone so iodine tracer preferred - hot nodule: rapid uptake of iodine or isotope, less likely to be malignant - cold nodule: little or no uptake of iodine or isotope: more likely to be malignant
61
Results of a radionuclide thyroid scan?
- 95% of solitary thyroid nodules are benign
62
What should you consider before doing a thyroid scan?
- if pregnant or breastfeeding - hx of allergies: iodine, shellfish, bee venom - meds: thyroid hormones, antithyroid meds, meds that contain iodine: cough syrup, multivitamins, and amiodarone
63
What will hot and cold nodules look like on a scan?
- hot: darker than rest of thyroid | - cold: can't see nodule, empty space
64
What is favored a single nodule or multi-nodular thyroid?
- multi-nodular is good finding
65
What is a fine needle aspiration (FNA)?
- non-surgical differentiation of malignant and benign nodules - cost effective, and safe - results are classified as: benign, follicular lesion of undetermined significance, follicular neoplasm, suspicious for malignancy, malignant, non-dx
66
Determining if nodular is benign or malignant with FNA?
- benign: consist of follicular epithelium with variable amount of colloid - malignant: can determine type: papillary, follicular variant of papillary, medullary, anaplastic, thyroid lymphoma, metastases to thyroid, * follicular CA and hurtle cell CA can't be dx clearly by FNA bx (have to go in surgically)
67
Suspicious findings from FNA
- 10% of FNAs - not clearly benign nor malignant - 25% are found to be malignant when undergoing thyroid surgery - end up being follicular or hurthle cell cancers - surgery is recommended for suspcious aspiration
68
Non-dx findings from FNA?
- cytologically inadequate - repeat FNA under US guidance - consider core-needle bx - if still non-dx surgical excision needs to be considered
69
How is thyroid function best assessed?
- by measuring serum TSH, assuming steady state conditions and absence of pituitary or hypothalamic disease
Endocrinology
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