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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