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123 Cards in this Set

  • Front
  • Back
what type of relationship exists between TSH and free T4 levels?
inverse log relationship - small alterations in free T4 produce a much larger response in serum TSH
what is the preferred screening test for evaluating thyrometabolic states, and why?
TSH test - excludes thyroid disease, detects disease, and indicates mild disease and thyroxine suppression in differentiated cancer patients
at what age are TSH concentrations high than normal?
in children, especially first week of life
what drugs decrease and increas TSH secretion (2,1)?
decrease: 1) dopamine; 2) glucocorticoids; increase: amiodarone
how do free T4 levels fluctuate in a euthyroid individual, and how does this relate to levels of binding protein?
the steady state level remains almost constant, and is independent of binding protein variations
what is the preferred type of test for measuring free T4?
why does a total T4 test not provide adequate clinical information?
changes in the concentration or affinity of serum binding proteins affect total T4
besides hyperthyroidism, what can high TT4 values reflect?
increases in thyroid hormone binding protein (and low TT4 levels may reflect decreases in thyroid hormone binding protein)
what can increase the amount of thyroid binding protein (examples)?
pregnancy, oral contraceptives, estrogen therapy, newborn, hereditary TBG increase
what can decrease the amount of thyroid binding protein (5)?
renal or GI protein loss, androgen therapy, glucocorticoid excess, cirrhosis, hereditary deficiency
what value should be reported with every total T4, and what is it?
free thyroxine estimate - an estimate of free T4, representing the total T4 corrected for abnormalities in the degree of serum protein binding
what are the units/range for FTI?
unitles, and has normal range close to normal range for total T4
when is FTI unreliable?
1) euthyroid patients with TBG excess or deficiency (congenital); 2) familial dysalbuminemic hyperthyroidism; 3) severe illness; 4) anti-T4 antibodies - this measurement is normal in hyper- and hypothyroid patients, healthy subjects, and subjects with mild abnormalities in binding proteins
what test was designed to correct for variations in protein binding of T4, and what are the units of this test?
T3 or T4 resin uptake test (T-uptake), expressed as THBR (thyroid hormone binding ratio)
how is this test used?
used with total T4 to calculate a free thyroxine index (FTI = TT4 * THBR)
what is a total T3 test useful in?
detection of T3-thyrotoxicosis - not a good general test
how is most T3 formed?
by conversion of T4 to T3 in peripheral tissues (80%), 20% is formed in thyroid
is most T3 protein bound?
yes, but the free fration is 10X as high as with T4
what is a free T3 test useful in?
also useful in diagnosing T3 thyrotoxicosis
what does an increased reverse T3, with low total T3 suggest?
nonthyroidal illness rather than hypothyroidism
what is reverse T3?
T3 with two iodines on the distal phenyl group (not the one with the NH2/COOH)
wheres is thyroglobulin stored and what does it serve as?
stored as colloid in thyroid follicle, serves as prohormone in thyroid hormone synthesis
what is measurement of Tg most useful in?
detection of residual/recurrent differentiated thyroid carcinoma following surgical resection and radioiodine ablation
what else is Tg useful for diagnosing?
1) diagnosing factitious hyperthyroidism (characterized by non-elevated serum Tg); 2) assessing the activity of inflammatory thyroiditis
what measurement is a sensitive and specific indicator of autoimmune thyroid diseease?
measurement of antibodies to thyroid peroxidase
where is the thyroid peroxidase enzyme located?
on the surface of the microvilli of thyroid epithelial cells
what other anti-thyroid andibodies can be tested for (2)?
1) antithyroglboulin (TgAb); 2) anti-TSH receptor antibodies (TRAb)
who has a high prevalence of TgAb?
adults without thyroid disease - it is less frequently positive in autoimmune thyroid disease than TPOAb
what problem can TgAb cause, and in who?
present in about 35% of thyroid cancer patients - these antibodies interfere with immunoassays for Tg
in what two diseases were these antibodies mentioned to be found?
1) Graves' disease; 2) Hashimoto's thyroiditis
what do these antibodies do in Graves' disease?
activate the TSH receptor, resulting in gland enlargement and thyroid hormone overproduction (test is also called TSI (thyroid stimulating immunoglobulins)
what does it do in Hashimoto's?
blocks the action of TSH, thus adccounting for the hypothyroidism
what populations with no overt signs or symptoms should undergo thyroid tests (3)?
1) newborns; 2) adults at 35, every 5 years; 3) elderly (high risk)
what are clues from lab results that there may be primary hypothyroidism (4)?
1) hyponatremia; 2) hypercholesteremia; 3) anemia; 4) elevated creatine kinase
what do low, normal, and high TSH values suggest about hypothyroidism?
normal values exclude primary hypothyroidism, high values suggest thyroid gland failure, and low values suggest secondary or tertiary hypothyroidism
what other test and value confirms primary hypothyroidism?
low free serum T4
what do slightly high TSH and normal free T4 values suggest?
subclinical hypothyroidism or recovery from nonthyroidal illness
what are the value of total and free T3 tests in hypothyroidism?
not indicated, since normal levels are found in 25% of patients with primary hypothyroidism
what test has 99% sensitivity for Hashimoto's?
TPOAb (99% in Hashimoto's, 90 to 95% for other autoimmune thyroid disease)
what are optimal screening tests for subclinical hypothyroidism (2)?
1) TSH; 2) TPOAb
after initiating or changing L-T4 replacement therapy, what happens to serum TSH?
6 to 8 weeks are needed for serum TSH to reach a new steady state%
what more reliably indicates new thyroid status during this period?
serum free T4
what should be measured, and how often, to monitor clinical euthyroidism (replacement therapy)?
TSH should be measured 1 to 2 times per year
what may low TSH levels raise the risk for (2)?
1) bone resoprtion (osteoporosis); 2) cardiac arrhythmias (especially atrial fibrillation)
what should be measured in patients suspected of noncompliance, and what values should these patients have?
TSH and free T4 - these values should be high in noncompliant patients
what test and result indicates secondary (central) hypothyroidism?
free serum T4 is used to make this diagnosis - it will be low
what will TSH be like?
subnormal, inappropriately normal (for low free T4), or sometimes elevated
why may it be elevated?
due to secretion of biologically inactive, but immunoreactive, TSH isoforms
what test differentiates between secondary and tertiary hypothyroidism?
TRH test - TRH is given, and if TSH doesn't increase, hypothyroidism is secondary to hypopituitarism - if there is a response, the problem is with the hypothalamus
what effect may mild hypothyroidism have on pregnancy?
can have detrimental effect - fetal loss and infant IQ decrease
what test is used for pre-pregnancy and first trimester screening?
serum TSH
what may be necessary for pregnant patients receiving L-T4 replacement therapy?
increased doses, to maintain normal TSH
***see flowchart on hypothyroidism***
what are laboratory value clues to hyperthyroidism (5)?
1) hypercalcemia; 2) hyperphosphatemia; 3) hyperglycemia; 4) hypercholesterolemia; 5) elevated alkaline phosphatase
what is the first test done to evaluate primary hyperthyroidism, and what level is observed in primary hyperthyroidism?
profoundly low/undetectable level (<0.01 mU/L) observed in primary hyperthyroidism - low level is also suspicious (<0.1 mU/L)
what does a normal level mean, and what can a high level be consistent with?
normal level excludes primary hyperthyroidism, and a high level is consistent with TSH-secreting tumor or reistance to thyroid hormones
what test and result confirms hyperthyroidism?
elevated free T4 level
what TSH and T4 levels may suggest subclinical hyperthyroidism, T3 thyrotoxicosis, or nonthyroidal illness?
low TSH level and normal level of free T4
what does a low T3 value suggest?
nonthyroidal illness
what does a normal T3 level suggest?
subclinical hyperthyroidism
what does a high T3 level suggest?
confirms T3 thyrotoxicosis
why may there be more T3 than T4?
autonomic toxic adenomas may produce more T3
what is the rationale for the radioactive iodine uptake test?
percent uptake of iodine into thyroid gland is proportional to rate of thyroid hormone production
what is the test used in?
differential diagnosis of biochemically established hyperthyroidism
what is the differential diagnosis for increased uptake (5)?
1) Graves' disease; 2) toxic multinodular goiter; 3) toxic autonomous nodule; 4) trophoblastic tumor; 5) TSH-secreting pituitary tumor
what is the differential diagnosis for decreased uptake (3)?
1) thyroiditis; 2) excessive thyroid hormone in medications or food; 3) iodine excess
what antibody do most patients with Graves' disease have, and what do fewer have?
most have TPOAb, fewer have TgAb
in what types of goiter (2) are no circulating antibodies seen?
solitary or multinodular toxic goiters
why are TSH receptor antibodies measured during last trimester of pregnancy?
to predict possible occurrence of neonatal or fetal thyrotoxicosis secondary to transplacental passage of maternal antibodies
in what conditions will high serum hCG be seen?
1) molar pregnancy; 2) choriocarcinoma; 3) hyperemesis gravidarum
what test is used to monitor effectiveness of antithyroid medication during the first 2-3 months of treatment?
serum free T4
what are serum TSH values useful for, and what are they like in early suppression therapy?
not an accurate measurement of thyroid status - often remains low for several months after normalization of T4 and T3 concentrations
what may overtreatment with suppression therapy lead to?
what is a marker for a TSH-secreting pituitary tumor, but not with resistance to thyroid hormone?
increase in the alpha-subunit of TSH (this increases with TSH secreting tumor, but not resistance to thyroid hormone)
what will happen when TRH is administered to patients with TSH-secreting tumors?
no increase in serum TSH
***see flowchart on hyperthyroidism***
what is NTI/sick euthyroid syndrome?
disorder characterized by thyroid hormone excess or deficiency in the absence of thyroid disease
what are some causes of euthyroid hyper- or hypothyroxinemia (4)?
1) acute/chronic illness; 2) alterations in binding proteins; 3) actions of drugs; 4) peripheral resistance to thyroid hormones
what is the clinical problem?
TSH and free T4 have reduced specificity for detecting thyroid dysfunction when patients are seriously ill, causing considerable diagnostic confusion
what might low levels of total and free T3 result from?
impaired conversion of T4 to T3 in peripheral tissues
what may raised levels of serum free T3 indicate, in what patients?
useful indicator of hyperthyroidism in a hospitalized patient
what was said about normal or low T3 in these patients?
does not rule out hyperthyroidism
what is serum free T4 like normally in sick euthyroid syndrome?
usually normal
what can be used for confirmation when a free T4 immunoassay result is abnormal in a hospitalized patient?
reflex total T4
in severe illness, what may total serum T4 be like?
what should one be suspicious of, if there is a low total T4 in a patient not in intesive care?
what may be true if both total and freee T4 are abnormal, in the same direction?
thyroid condition may be present
what if the total and free T4 are discordant?
thee free T4 abnormality is probably not due to thyroid dysfunction, but is more likely a result of the illness, medications, or an artifact of the test
what medications (2) were said to directly inhibit pituitary TSH secretion?
1) dopamine; 2) glucocorticoids
what are serum TSH levels like in most euthyroid sick patients?
what may serum TSH levels be like in acute illness?
mildly suppressed
when may TSH be elevated in NTI?
during the recovery phase (transient rebound effect)
what will TSH reference level be like in the hospitalized setting?
widened (0.05 - 10.0 m/UL)
what is the most useful test combination to detect thyroid dysfunction in a sick hospitalize patient?
TSH plus free T4
when should diagnostic testing be done, whenever possible?
deferred until the illness has resolved
what is the exception?
when patient's history or clinical features suggest the presence of hyroid dysfunction
what type of cancer is serum TSH considered a growth factor for?
DTC (differentiated thyroid carcinoma)
what test should suppress TSH to subnormal values?
what happens ot serum TG following successful therapy of DC?
falls to undetectable levels
what does presence of Tg after thyroidectomy and ablation indicate?
pathognomonic for the presence of residual or recurrent tumor
what can interfere with immunoassays for Tg, and cause falsely low or high serum Tg values?
what misjudgement of Tg (from TgAb) is a seroius concern?
underestimation is particuarly serious, and has potential to mask metastatic disease
what do serum measurements of TgAb have prognostic value for monitoring, and what levels indicate what problem?
the treatment of TgAb DTC patients - increasing levels are often the earliest sign of residual or recurrent tumor
what test is used to detect cells containing Tg mRNA, and what may this test be useful in doing?
RT-PCR (reverse transcriptase PCR) - may help overcome limitations of current Tg/TgAb methods
what is used to identify patients with familial medullary thyroid carcinoma?
RET proto-oncogene analysis
what other patients can be identified with this method?
those with MEN II and III
what is used less frequently to diagnose MTC?
measurements of serum calcitonisn
what is thyroid storm, how is it diagnosed, and what is the mortality (with treatment)?
a life-threatening condition characterized by an exaggeration of the manifestations of thyrotoxicosis - diagnosis is clinically based, 30% mortality even with treatments
what is myxedema coma, and when is treatment initiated?
a life-threatening condition characterized by exaggerated hypothyroidism - treatment is initiated without waiting for lab reults