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Thyroid and Thyroid Testing
Edgar Betancourt, MD
Aspirus Wausau Hospital
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Where is it?
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Histologic Section of Normal
Thyroid
colloid Follicular cells
C cells
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What Does It Do?
Sole source of thyroid hormone (T3&T4)
Thyroid hormone is essential in
development
In conjunction with other endocrine
hormones is essential to the regulation of
body metabolism
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Thyroid Hormone Regulatory
Pathways
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Thyroid Hormone Regulatory
Pathways As in most endocrine systems the main
regulatory hormone, TSH is produced by
the pituitary gland.
The hypothalamus produces TRH which in
turn induces the pituitary to produce TSH
Secretion of TSH and TRH are inhibited bya negative feedback loop by T3 & T4
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Thyroid Hormone Physiology
T4 is secreted in a 20 fold excess to T3
T3 is the most active fraction
After secretion the hormone is rapidly
bound to plasma proteins (>99%):
Thyroid binding globulin (TBG)
Transthyretrin (Prealbumin)
Albumin
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Thyroid Hormone Physiology
T4 to T3 conversion happens both before
and after secretion.
Only the unbound fraction (free) is
metabolically active.
Thyroid hormone receptors are ubiquitous
nuclear receptors in all tissues and lead topromotion or inhibition of transcription
depending the particular cell.
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Thyroid Disease Two conditions are possible hypo or
hyperthyroidism
Congenital hypothyroidism leads to the
syndrome known as Cretinism characterized
by short stature and mental retardation.
Since the disorder is preventable neonatalscreening is performed
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Thyroid diseases
Most common in Females (4:1)
Associated with anti TPO antibodies
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Dont Blame this on
hypothyroidism!
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Thyroid Disease Hyperthyroidism
Usually autoimmune, characterized by
antibodies against the TSH receptor which
lead to receptor activation.
One tenth as common as hypothyroidism.
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Thyroid Disease
Graves disease is the most common syndrome
Irritability
Heat intolerance
Palpitations
Weight loss
OphthalmopathyGoiter
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Thyroid Disease
Exophthalmos
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Thyroid TestingFinally
A little history.. (before TSH)
Only total T3, T4 and TBG were widely
available
Total T3 and T4 varied widely particularly
depending on TBG levels.
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Thyroid Testing
Total T3 and T4 are not accurate
measurements of the thyroid axis
metabolism.
Values vary widely due to Binding protein
levels as seen pregnancy, nephrotic
syndrome, etc.
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Thyroid Testing TSH to the Rescue!
TSH was established as prime indicator of
thyroid axis homeostasis.
The initial problem with TSH was
technological, assays did not have adequate
sensitivity to accurately measure analytes inthe micromolar range
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Thyroid Testing
Once sensitive immunoassays were
established only one test could evaluate
whether the patient was hypo, hyper oreuthyroid.
Additional tests, Anti TPO, free T4 and free
T3 could then elucidate the most likelycause of hyper or hypo function.
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Thyroid TestingHypothyroid
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Thyroid TestingHyperthyroid
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Thyroid Testing
Thyroid testing is a classic example of a test
that best fits the Reflex test model.
Instead of the clinician ordering from a
battery of different tests only 2 orders are
necessary:
Thyroid Function Cascade (for diagnosis)TSH (for monitoring)
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Thyroid Testing
The cascade testing model saves time,
money and resources and provides a
standardized method for thyroiddysfunction evaluation.
Transforms the laboratory from a simple
provider of analytical data to aconsultational service.
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Thyroid Testing
One caveat!
Thyroid testing should not be performed on
severely ill patients. During severe illness
physiologic alterations to baseline thyroid
homeostasis are common as a response to
the acute illness.
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Thank You!
Meyers Beach, CornucopiaWisconsin
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