Regional Laboratory
AllianceRegional Laboratory
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Physician organizations such as the American College of Physicians and the US Preventive Services Task Force do not recommend screening the general adult population for occult thyroid disease because it has a low yield and is not cost effective. However, they do recommend selective testing of women aged 50 years and older with nonspecific symptoms and all women over 60 years. The College of American Pathologists recommends ordering a TSH for the following patient populations:
Among adults over age 50, it is estimated that unsuspected hypothyroidism will be found in up to 7 men and 18 women per 1000 persons tested. In addition, about five of 1000 older women will be found to have unsuspected hyperthyroidism. In the geriatric population, in whom symptoms are often difficult to attribute to any single etiology, up to 40 cases of thyroid disease will be found per 1000 persons tested.
Detection and treatment of early thyroid disease offers three potential health benefits. First, progression to overt hypothyroidism, which develops at a rate of 5 to 25% per year in patients with anti-thyroid antibodies, may be prevented. Second, serum cholesterol levels in patients with hypercholesterolemia induced by thyroid failure may be reduced by treatment with levothyroxine. Third, unrecognized symptoms of thyroid hormone deficiency that diminish quality of life and consume medical resources may be reversed much sooner. Based on these potential benefits, a recent study concluded that measuring serum TSH at the periodic health examination in adults over the age of 35 years was as cost effective as other widely accepted preventive medical practices such as screening for hypertension (JAMA. July 24/31, 1996; 276: 285-92). Repeating TSH measurements at five year intervals was most cost effective.
Thyroid function tests are among the most common laboratory tests. They account for 4% of all outpatient laboratory tests and 8% of laboratory charges. Traditionally, at least two thyroid function tests, thyroxine (T4) and T3 uptake (T3U), were ordered on each patient. The product of these tests was also reported as the free thyroxine index (FTI) or T7. Improvements in thyroid testing has largely eliminated the need for these traditional tests.
New methods for direct measurement of free T4 have eliminated the need to estimate free hormone levels with the T3U and FTI. The introduction of sensitive thyrotropin (TSH) assays has transformed thyroid function testing from a thyroxine to a TSH based strategy. The American Thyroid Association recommends the combined use of TSH and free T4 as the most efficient combination of blood tests for diagnosis and follow-up of both ambulatory and hospitalized patients. Today, the preferred method of testing for thyroid disease is a thyroid test cascade, starting with a TSH assay. If TSH is normal, no further tests are performed. If TSH is abnormal, a free T4 level is automatically performed (see accompanying algorithm).
A retrospective study of all TSH and FT 4 tests performed on 2629 adult outpatients at UCSF during a 6 month period demonstrated the merit of this approach (Arch Intern Med 1996;156:2333-37).
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Seventy percent (1835/2629) of all TSH results were normal. Of the 1835 patients with normal TSH levels, only 1.9% had an abnormal FT4; 11 had low and 24 had high FT4 levels. No thyroid disease was detected in any of the patients with normal TSH and low FT4 levels. Of the 24 patients with normal TSH and high FT4, all were being monitored for thyroid replacement , thyroid suppression or amiodarone therapy. None of the elevated FT4 levels led to a new diagnosis or altered treatment.
Thirty percent (794/2629) of patients had abnormal TSH values. In three fourths of this population, FT4 results were concordant with regard to thyroid function (e.g. low TSH - high FT4 or high TSH -low FT4). Most of the cases with discordant results involved abnormal TSH and normal FT4 levels. For example, 76% of patients with low TSH values had normal FT4 levels. Similarly, 89% of patients with high TSH values had normal FT4 levels. In each of these cases subclinical hyperthyroid or hypothyroid disease was diagnosed. TSH was more sensitive in detecting subclinical thyroid dysfunction.
Total laboratory charges for thyroid tests at UCSF were just over $1 million per year. If FT4 had been ordered only when TSH levels were abnormal, approximately 36,000 FT4 tests would have been avoided. Eliminating these tests would save approximately $210,000 per year in laboratory charges.
This study involving outpatients at a large medical center demonstrated that TSH and FT4 were frequently ordered together, but this practice was not necessary to achieve diagnostic accuracy. When TSH levels were normal, FT4 levels were normal 98% of the time. TSH, by itself, should be ordered to assess thyroid function in outpatients. FT4 should be ordered only when TSH is abnormal. This practice will eliminate unnecessary testing and result in substantial cost savings.
RLA OFFERS CONTINUING EDUCATION SERIES
Enclosed with this letter and algorithm you will find your invitation to the first in a series of continuing education programs sponsored by the RLA. We hope you will be able to join us!