Posted January 1, 2005
Hyperthyroidism and hypothyroidism are common conditions that have
lifelong effects on health. About 5% of U.S. adults report having
thyroid disease or taking thyroid medication at some time. Thyroid
dysfunction, which can be diagnosed by thyroid function tests, is
usually readily treatable. Clinical manifestations are often insidious
and vary considerably among patients. If symptoms alone were relied
on for diagnosis, many patients would go untreated. Consequently,
physicians are more often excluding hypothyroidism or hyperthyroidism
through routine laboratory screening, rather than establishing thyroid
disease as their primary diagnosis. The goal of screening is to identify
patients with subclinical thyroid dysfunction and to treat them before
they develop symptoms and complications of their disease.
Serum thyroid stimulation hormone (TSH) measurement is the single
most reliable test to screen for and to diagnose all common forms
of hypothyroidism and hyperthyroidism. Most labs now use third-generation
TSH assays, which have detection sensitivity of less than 0.02 mU/L.
These assays generally show a specificity of greater than 99% and
positive predictive values over 96%. If less sensitive assays are
employed, a free-T4 assay and a total or free-triiodothyronine (T3)
assay should be performed at the time of screening to reliably distinguish
hyperthyroid patients from euthyroid patients. About 85% of ambulatory
patients without associated disease or pituitary dysfunction will
have normal TSH values and do not require further testing.
In both subclinical (normal free-T4) and clinical hypothyroidism
(decreased free-T4), the TSH will be elevated. The only exception
to this is secondary (central) hypothyroidism (e.g. TSH secreting
pituitary adenoma or thyroid hormone resistance), which only accounts
for about 1% of the cases. Primary thyroid disease accounts for over
99% of the cases of hypothyroidism. In secondary hypothyroidism the
TSH is usually normal, but may be low or even mildly elevated. If
secondary disease is suspected, a free-T4 must be performed along
with TSH. If subclinical or clinical hypothyroidism is present, testing
for thyroid peroxidase antibodies (TPOAb) should be performed to
rule out an autoimmune mechanism (e.g. Hashimoto thyroiditis).
Virtually all commonly encountered types of hyperthyroidism show
suppressed serum TSH, typically less than 0.1 mU/L. Serum free-T4
measurement is indicated to further assess the severity of hyperthyroidism
in those patients with a serum TSH level less than 0.1 mU/L. If the
serum TSH is decreased and the free-T4 is normal, total or free-T3
must be performed to rule out T3 toxicosis. If Graves’ disease
is suspected, testing for TSH receptor antibodies (TRAb) should be
performed.
The American Thyroid Association and the U.S. Preventative Services
Task Force have advocated screening asymptomatic adults with serum
TSH at age 35 years and every 5 years thereafter, but the cost-effectiveness
of this is still controversial. More frequent screening may be appropriate
in individuals at higher risk of developing thyroid dysfunction.
Screening of newborn children for hypothyroidism is already a widely
accepted and legislatively mandated practice.
There are effective therapies for both hypothyroidism and hyperthyroidism
for patients in whom treatment is indicated, such as in high-risk
patients with, for example, hyperlipidemia, atrial fibrillation or
reduced bone mineral density. It is still uncertain whether treatment
will improve the quality of life in otherwise healthy patients who
have subclinical thyroid disease.
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NewsPath® Editor: Megan
J. DiFurio, MD, FCAP
This newsletter is produced in cooperation with the College of American
Pathologists Public Affairs Committee and may be reproduced in whole or
in part as a service to the medical community. Copyright © 2006 by
the College of American Pathologists.
Please e-mail any comments to newspath@cap.org.
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