Thyroid function lab test Flashcards
what are the thyroid hormones?
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
What form of T3 and T4 can penetrate cellular membranes and exert biologic activity by interacting w/ nuclear receptors?
-only free T3 and T4,
Where is T4 converted to T3?
especially in the liver, gut, skeletal muscle, brain, and thyroid
What are the thyroid function tests?
- 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)
What is the best assessment of Thyroid function?
TSH–3rd generation assay is the most sensitive!
What disease are the following abys associated with?
- TPO-Ab (thyroglobulin aby)
- Tg-Ab
- TSH receptor stimulating ab
- TSH receptor blocking ab
TPO: Hashimotos
Tg-Ab: Hashimotos
TSH Stimulating: Graves
TSH Blocking: Hashimotos and sometimes Graves
TSH testing is used to…
- dx thyroid disorder in symptomatic patient
- screen newborns for underactive thyroid
- monitor thyroid replacement therapy in ppl w/ hypothyroidism
How does TSH vary with age?
Higher TSH conc in older patients.
When might you see an elevated TSH? Low TSH?
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
What is the most prominent thyroid hormone? When is this hormone active?
T4, greater than 90%, nearly all of it is transported bound to proteins (TBG, albumin)
-only free(unbound) T4 is metabolically active
WHen would you see increased T4 levels? Decreased?
Increased:
- hyperthyroidism
- acute thyroiditis
- pregnancy
- increased TBG (Thyroid binding globulin)
Decreased:
- hypothyroid states
- pituitary insufficiency
- hypothalamic failure
- protein malnutrition
- iodine insufficiency
How migh levels of TBG change the value of total T4?
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
What does Free T4 Index measure? What does T3 resin uptake measure?
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.
Measurement of Free T4 is indicated when…
- dx hypo/hyperthyroidism
- monitor resposne to therapy along with TSH
- Gives quicker result to response to therapy with replacement thyroxine then TSH
what % of thyroid hormone is T3?
When might T3 be increased? Decreased?
- 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
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?
-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.
when is free thyroxine index (FTI or T7) used?
-evaluates thyroid function in patients with protein abnormalities (low albumin, lor or high TBG)
What are the most common thyroid autoantibodies?
Indications for testing for these abys?
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
Thyroid Stimulating Hormone abys
- what are they?
- can these cross the placenta?
- what disease is most likely if you have TSH stimulating/blocking ab?
- immunoglobulins that stimulate or inhibit thyroid hormone release
- crosses placenta
Stimulating aby-graves diseasee (90% sensitivity)
blocking aby- hashimotos thyroiditis
What is Thyroglobulin (Tg)?
-when might this be elevated?
protein precursor of thyroid hormones, may be used as a tumor marker.
- elevated with:
- -thyroiditis
- -graves disease
- -thyroid cancer
What are the initial labs to establish hypo/hyperthyroidism?
What helps determine if true elevation or depression in T4 or T3?
- inital labs: TSH and Free T4
- FTI (free thyroxine index) helps determine depression or elevation in T4 or T3
Thyroid Cancer:
- most common in what ages?
- what predisposing conditions?
- 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.
IF you find a thyroid Nodule what steps will you take in dx ?
- thorough hx
- measure TSH; if low usually indicates probable over hyperthyroidism, if high more suspicious of cancer*
-THyroid US; confirms nodule presence.
Thyroid US
- good at indicating what?
- characteristics of a benign nodule?
- deciding when to do a FNA biopsy
- 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
Radionuclide Thyroid Scan
- what is this?
- why do this?
-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
Interpreting a radionuclide Thyroid scan
- “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
FNA (Fine needle AspiratioN)
- what does this determine?
- results are classified as?
- what two CA cannot be diagnosed clearly by FNA biopsy?
-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.
what does the result of “non-diagnostic” mean of a FNA bx?
-cytologically inadaquate, need core-needle bx or repeat FNA under US guidance.
