THYROID FUNCTION TESTS



Serum Thyroid Hormone Concentration. No one test can specifically determine thyroid status. Thus a variety of tests have been developed. These must be interpreted in an integrated fashion, keep­ing the patient’s clinical presentation clearly in mind. Measurement of serum total T4 (5 to 11 u.g/ dl) and free T4 concentration (1.5 to 3.5 ng/dl) is the best current initial screening mechanism for thyroid dysfunction. Since the serum total T4 can be altered by changes in carrier-protein concentration as well as by changes in thyroid function this value must be evaluated in conjunction with aTnieasurement of serum free T4 or an indirect index of free T4. Direct measurement of serum free T is preferable. Many centers, however, continue to4 use indirect methods and derive a “free T4 index” (0.5 to 1.5) from a combination of serum total T4 and resin T3 uptake test (RT3U). The latter procedure allows an estimation of changes in serum TBG. The patient’s serum, a trace amount of radioiodine-labeled T3, and an anion-exchange resin are co-incubated. The resin-bound T3 is measured and expressed as a percentage of the total amount of tracer T3 added (25 to 35 per cent). Values correlate inversely with the concentration of unsaturated TBG. Generally, a parallel increase or decrease in both total T4 and RT3U indicates hyperthyroidism or hypothyroidism, whereas op­posite changes suggest alterations in TBG binding. The derivation of the free T4 index from the total T4 and T3 uptake generally gives a normal value in the setting of alterations in TBG. The free T4 index or absolute free T4 concentration is high or low in the hyperthyroid or hypothyroid subject, respectively. Occasionally free T4 values may be misleadingly low in patients with severe nonthy-roidal illness, “euthyroid sick syndrome.” Serial analysis of thyroid function tests will generally resolve this dilemma. Measurement of total and free T3 values is clinically useful only when T3-thyrotoxicosis is suspected. In this setting T4 val­ues are normal and the hyperthyroidism will be missed in the absence of a T3 measurement. Mea­surement of serum rT3 has limited value as a rou­tine clinical test. A measurement of serum TSH is the single best test to detect primary thyroid failure, since the value is invariably elevated in primary hypothyroidism owing to loss of the in­hibitory feedback effect of thyroid hormone. Cur­rently the majority of radioimmunoassays for TSH cannot differentiate between a normal TSH and the suppressed value found in hyperthyroidism.
The TSH response to TRH is a very useful test. The normal TSH response ranges between a serum level of 15 and 30 u.U/ml at 20 minutes after 500 (xg of intravenous TRH. An incremental TSH rise of 5 mU above baseline in response to TRH is con­sistent with a normal axis. In hyperthyroidism the TSH response is flat. The mildly hypothyroid pa­tient with a borderline high serum TSH value will have an exaggerated response to TRH.

Thyroid Radioiodide Uptake (RAW) Test The accumulation of tracer iodide (1231,131I) in the thy­roid can be quantitated at standard intervals (6 to 24 hours) to assess thyroid activity. Owing to the enrichment of western diets with iodide, this pro­cedure is of no value in the differentiation be­tween a normal and a hypoactive gland. In ad­dition the uptake value may be normal in many hyperthyroid patients (approximately 30 per cent), especially with multinodular goiter and with T3 thyrotoxicosis. Consequently a normal RAIU test does not exclude hyperthyroidism. In patients with thyrotoxicosis associated with thy­roiditis, low (<3 per cent) uptake values are usually found in contrast to normal or high values in M Graves’ disease. A low uptake value is also a fea- I ture of thyrotoxicosis consequent to excess thy- 1 roid hormone medication.

Immunologic Tests. Autoimmune thyroid dis- -ease (Hashimoto’s thyroiditis and Graves’ disease) is associated with positive tests for antibodies to specific thyroid antigens (antimicrosomal and an-tithyroglobulin), which are useful in the detection of these autoimmune disorders. The recent avail­ability of a biological assay for thyroid-stimulat­ing immunoglobulin (TSI), a specific marker for Graves’ disease, has helped in both the diagnosis and management of this common disorder. A re­duction in TSI level during therapy with antithy­roid drugs is a good indicator that a remission has been induced and that therapy can be modified or withdrawn.

Scintiscan. The RAIU test provides useful in­formation on the anatomy of the thyroid. A scan at 6 hours demonstrates a homogeneous distri­bution of 123I in the normal gland, whereas 123I activity will be heterogeneous in Hashimoto’s thyroiditis and multinodular goiter. Furthermore, this procedure will differentiate a functioning (be­nign) from a nonfunctioning (possible carcinoma) thyroid nodule. Functioning nodules demonstrate normal (warm) to increased (hot) 123I activity, whereas nonfunctioning nodules fail to concen­trate 123I (cold).

Sonography. Ultrasound imaging will decisively differentiate whether a nonfunctioning nodule on 123I scan (cold nodule) is cystic or solid. In ad­dition, this noninvasive procedure is very helpful in the serial sizing of the thyroid nodule and pro­vides an objective basis for the evaluation of T4 suppressive therapy.

Needle Biopsy. Fine needle aspiration biopsy with cytological analysis of the tissue has now become a routine procedure in the evaluation of a thyroid nodule. The specificity and sensitivity of this procedure are now so reliable that most centers will use needle biopsy as the first step in the evaluation of a thyroid nodule, even before scintiscanning.





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