Interpretation of laboratory thyroid function tests for the primary care physician. (Featured CME Topic: Thyroid Dysfunction/Disease).

THYROID FUNCTION TESTS are frequently ordered in both the inpatient and outpatient settings. A basic knowledge of thyroid-hormone physiology is helpful in interpretation of laboratory results.

Thyroxine ([T.sub.4]) is released from the thyroid gland along with small amounts of triiodothyronine ([T.sub.3]) and thyroglobulin under the guidance of thyroid-stimulating hormone (TSH). Secretion of TSH is principally regulated by circulating levels of thyroid hormones (via a negative-feedback loop) and hypothalamic thyrotropin-releasing hormone (TRH). More than 99% of the thyroid hormones are bound to proteins, including thyroid-binding globulin (TBG), thyroxine-binding prealbumin, and albumin. Less than 1% are free, unbound hormones that make up the biologically active fraction, and they are not generally influenced by thyroid-binding protein abnormalities. Thyroxine is tightly bound to TBG, whereas [T.sub.3] is less tightly bound to TBG but more tightly bound to thyroxine-binding prealbumin and albumin. The majority of serum [T.sub.3] levels (over 75%) come from peripheral conversion of [T.sub.4]. In nonthyroidal illness, [T.sub.4] conversion to [T.sub.3] is reduced and conversion to reverse [T.sub.3] (r[T.sub.3]) is enhanced.

SELECTION AND INTERPRETATION

A selection of thyroid function tests currently available in most laboratories is shown in Table 1.

Serum TSH Level

The American Thyroid Association recommends initially checking free-[T.sub.4] and TSH levels to test thyroid function. (1) Serum TSH level remains the single best test of thyroid function. Thyroid-stimulating hormone testing is the preferred approach because: 1) TSH is central to the negative-feedback system; 2) small changes in serum thyroid function cause logarithmic amplification in TSH secretion; and (3) the most advanced (third-generation) chemiluminescent TSH assays can now detect both elevation and significant lowering of TSH levels, and are capable of reliably measuring values <0.1 mU/L, thus aiding detection of subclinical thyrotoxicosis. In many situations, a normal TSH level can be sufficient indication to halt further testing of thyroid function; however, it may still be desirable to check a free-[T.sub.4] level in the setting of possible hypothalamic pituitary disease. In patients with central hypothyroidism, regulation of [T.sub4] replacement may need estimations of free-[T.sub.4] levels, since TSH levels may be less reliable.

Total Serum [T.sub.4] Level

Total serum [T.sub.4] level, measured by radioimmunoassay, has high sensitivity in reflecting the functional state of most patients with thyroid disease. The levels are high in approximately 90% of hyperthyroid patients and low in approximately 85% of hypothyroid patients. (2)

Measurement and Estimation of Free-[T.sub.4] Level

Free-[T.sub.4] level can be estimated by several different methods. It can be measured directly by equilibrium dialysis techniques or estimated indirectly by calculation of the free-thyroxine index (FTI) as follows:

Free-thyroxine index (FTI) = [T.sub.4] * x [T.sub.3] resin uptake ([T.sub.3]RU) %.

* [T.sub.4] refers to total serum thyroxine level.

The [T.sub.3] resin uptake ([T.sub.3]RU) test indirectly estimates unsaturated binding sites on TBG.

The patient's serum is incubated with radiolabelied [T.sub.3] tracer. The unbound tracer is trapped with resin, and the value is reported as percent tracer uptake by resin. The greater the number of free TBG-binding sites, the lower the uptake of tracer by the resin. The normal range for [T.sub.3]RU is between 25% and 35%. The factoring of [T.sub.3]RU along with total-[T.sub.4] level in the FTI calculation is useful in interpreting alterations of serum total-[T.sub.4] level with variations of TBG. This is illustrated by the following example:

A 40-year-old female taking estrogen in the absence of intrinsic thyroid disease will have normal free-[T.sub.4] and TSH levels. She will, however, have an elevated total-[T.sub.4] level due to the increased TBG from estrogen exposure. The [T.sub.3]RU will be decreased due to the increased availability of hormone-binding sites on TBG, making less hormone available for uptake by the resin. Thus, despite an elevated total-[T.sub.4] level, the decreased [T.sub.3]RU will keep the FTI within normal limits.

Because of TBG level abnormalities, which require [T.sub.3]RU for darification, many laboratories routinely check both total-[T.sub.4] level and [T.sub.3]RU, and report the result as the FTI. (3)

Many factors can increase [T.sub.4] levels in the absence of a hyperthyroid state. A summary of the causes of euthyroid hyperthyroxinemia is listed in Table 2. (4)

Possible Errors in FTI Calculations

There are, however, binding-protein abnormalities that result in erroneous calculations of FTI. Familial dysalbuminemic hyperthyroxinemia (FDH) is an autosomal dominant condition with a prevalence of 0.2% in the Hispanic population. Individuals with this condition have a mutant albumin with a high capacity for binding [T.sub.4], but not [T.sub.3]. These individuals are euthyroid but present with a high [T.sub.4] level and have a high [T.sub.3]RU. The elevated [T.sub.3]RU results from normal binding of free [T.sub.3] to resin, but not to the abnormal albumin. Thus, the FTI is elevated, suggestive of thyrotoxicosis, but the normal TSH level should raise doubt regarding this diagnosis. The diagnosis of FDH can be confirmed by rechecking [T.sub.3]RU, using [T.sub.4] instead of [T.sub.3] tracer, and by checking thyroid function in relatives, or by electrophoretic studies of binding proteins.

Estimation of Serum [T.sub.3] Level

Serum total-[T.sub.4] and total-[T.sub.3] values reflect not only hormone production, but also the serum concentrations of thyroid hormone-binding proteins. In the presence of elevated levels of binding proteins, serum total-[T.sub.4] and total-[T.sub.3] concentrations are usually increased, but serum free-[T.sub.4] and free-[T.sub.3] concentrations remain normal.

The main purpose of free-[T.sub.4] and free-[T.sub.3] assays is to distinguish reliably between thyrotoxicosis, hypothyroidism, and the euthyroid state. This objective that cannot be attained with assays of total-[T.sub.4] and [T.sub.3] because of hereditary and acquired variations in the concentrations of binding proteins. (5)

Serum [T.sub.3] estimations are useful where free-[T.sub.4] levels are normal in the setting of suppressed TSH levels. Although in most instances a total-[T.sub.3] level is satisfactory, in the presence of a normal total-[T.sub.3] level, a free-[T.sub.3] level will distinguish [T.sub.3] toxicosis from subclinical thyrotoxicosis. (6) Suppressed TSH level with low to low-normal level of free-[T.sub.4] is also seen in factitious thyrotoxicosis induced by [T.sub.3] supplementation.

Triiodothyronine levels can be misleading in euthyroid patients with acute illnesses, such as cirrhosis, uremia, or malnutrition. Diminished peripheral conversion of [T.sub.4] to [T.sub.3] contributes to low serum-[T.sub.3] levels. [T.sub.3] levels are low in only 50% of hypothyroid patients. Relatively more [T.sub.3] than [T.sub.4] is produced as the thyroid fails.

Measuring [T.sub.3] levels during treatment with antithyroid medication may have predictive value in the management of Graves' disease. Estimating serum-[T.sub.3] levels may also be useful in amiodarone-induced thyrotoxicosis, and in assessing recurrence after antithyroid therapy. (7) The ratio of [T.sub.3] to [T.sub.4] is generally higher in patients with Graves' disease versus patients with thyroiditis or iodide-induced thyrotoxicosis. (8) Moreover, a ratio of >20 ng/[micro]g during antithyroid therapy for Graves' disease may indicate remission is less likely. (9)

Measurement of reverse [T.sub.3] (r[T.sub.3]) is not widely available. When r[T.sub.3], [T.sub.3], and [T.sub.4] levels are all elevated, it can be assumed that there is overproduction of thyroid hormone. An elevated r[T.sub.3] level, low level of [T.sub.3], but normal [T.sub.4] level is seen in euthyroid patients who are sick. (10)

ANCILLARY TESTS

Ancillary tests, such as those for thyroid-stimulating immunoglobulin (TSI) levels and antithyroid antibody levels are useful in the management of Graves' disease and Hashimoto's disease, respectively.

The most common antithyroid antibodies (antimicrosomal/peroxidase and antithyroglobulin) are highly organ-specific and organ-sensitive. The antimicrosomal antibodies, directed primarily against membrane-bound thyroid peroxidase, are most useful in diagnosing Hashimoto's thyroiditis. Tests for these antibodies may also be positive in Graves' disease.

More than 90% of patients with Graves' disease have an immunoglobin G (IgG) antibody, TSI, directed against the thyroid TSH receptor. TSI level determination is unnecessary in most cases, due to characteristic clinical manifestations of Graves' disease. Determining the TSI level may be of value in establishing the diagnosis of euthyroid Graves' disease in patients with ophthalmopathy. Thyroid-stimulating immunoglobulin levels have been shown to correlate directly with activity and severity of the thyroid-associated ophthalmopathy. (11) The combination of absent antithyroperoxidase antibody and high TSI levels in Graves' disease appears to be associated with a markedly increased risk of clinically evident ophthalmopathy. (12)

Serum thyroglobulin levels may have utility in distinguishing Graves' disease from factitious thyrotoxicosis. In Graves' disease, the level of serum thyroglobulin is increased, whereas in factitious disease the levels are decreased. Testing the serum thyroglobulin level is a method frequently used to detect recurrence of differentiated thyroid cancer. It may be elevated in many benign thyroid diseases, such as subacute thyroiditis. It has no use in separating benign thyroid disease from malignancy.

The test for thyrotropin-releasing hormone (TRH) has been almost completely replaced by more preferred immunoradiometric and other supersensitive TSH assays. (13) The use of TRH to distinguish between hypothalamic and pituitary causes has been questioned. (14,15)

OPTIMAL INTERPRETATION

Optimal interpretation of thyroid function calls on the clinician to consider many factors (Table 3).

Confirming abnormal thyroid function by repeating the laboratory tests is often an essential initial step. Rarely, a decimal-point error can occur when the data is entered into the computer.

The occupation of the patient is helpful and may raise the possibility of factitious thyrotoxicosis. Rarely, medical laboratory workers mayz have antibodies to mice immunoglobulins, which could result in artificial elevation of TSH levels in some assays.

A patient's medical and family history may be helpful in making a correct diagnosis. Clinical correlation in the presence of abnormal thyroid function is essential. The presence of antibodies in some autoimmune diseases could interfere with thyroid hormone assays. Discussion with laboratory personnel regarding a specific assay is often helpful before considering alternate testing.

The presence of clear biochemical thyrotoxicosis in the absence of thyroid enlargement or other physical findings noted with hyperthyroidism may be seen with factitious thyrotoxicosis. Many medications apart from thyroid hormone replacement and antithyroid medications influence thyroid function (Table 4). Many patients also take health food supplements, such as tyrosine, kelp, iodine, and possibly thyroid extract, that may influence results of thyroid tests.

Considering the relative deviation of TSH versus serum thyroid hormones may offer useful insight. Considering common etiologies first will enable a cost-effective work up by the generalist. An algorithm which may be useful in interpreting thyroid function is provided in the Figure.

Continued observation, with periodic monitoring for mild or insignificant abnormalities and endocrine consultation for significant abnormalities, is an option for the primary care physician.

TABLE 1.

Thyroid Function Tests Listed According to Estimated Cost


$70 - $100  Thyroid-stimulating immunoglobulin
$50+        Thyroid antibodies:
             thyroperoxidase Ab, thyroglobulin Ab
$30+        Free [T.sub.3]; thyroid-stimulating
             hormone; serum thyroglobulin
$20+        Total [T.sub.3]
$10+        Free [T.sub.4] total [T.sub.4]; [T.sub.3]
             resin uptake
TABLE 2.

Euthyroid Hyperthyroxinemia (4)


Increased Total-[T.sub.4] and Normal Free-[T.sub.4]
  Levels Thyroid-binding globulin:
    Estrogen exposure, medications (tamoxifen,
    raloxifene, clofibrate, 5-fluorouracil,
    perphenazine, methadone), pregnancy, acute
    liver dis
  Thyroxine-binding prealbumin:
    Inherited, pancreatic neuroendocrine tumors
  Albumin:
    Familial dysalbuminemic hyperthyroxinemia
  [T.sub.4] antibody-associated hyperthyroxinemia
Increased Total-[T.sub.4] and Increased Free-[T.sub.4]
  Levels
  Thyroid hormone resistance
  Medications (amiodarone, beta-blockers, oral
  contrast media) [T.sub.4] therapy
  Thyroid stimulation:
    Hyperemesis gravidarum, acute psychiatric illness
Normal Total-[T.sub.4] and Increased Free-[T.sub.4] Levels
  Acute severe illness
Miscellaneous
  High altitude
  Hyponatremia
TABLE 3.

Options to Consider in Interpreting Thyroid Function


1. Repeat laboratory test (consider
     laboratory error, wrong patient, etc)
2. Clinical correlation/status
3. Patient occupation, past medical history, family history
4. Medications
5. Consider alternate tests
6. Consider common etiologies first
7. Endocrine consult
TABLE 4.

Drugs Affecting Thyroid Function

            Effect                              Drugs

May cause hypo-                  Lithium, iodine (all forms, inclu-
 thyroidism                      ding kelp, contrast media, etc),
                                 interleukin-2, interferon-alpha
May cause hyper-                 Iodine, interleukins, interferons
 thyroidism
Reduce conversion of             Glucocorticoids, iodine, propylthio-
 [T.sub.4] to [T.sub.5]          uracil, beta-blockers, amiodarone
Suppress thyroid-                Dopamine, dobutamine, glucocor-
 stimulating hormone             ticoids, phenytoin, bromocriptine,
                                 octreotide
Increase clearance of [T.sub.4]  Carbamazepine, phenytoin,
                                 rifampin, phenobarbitol
Reduce binding of                Salsalate, salicylates, nonsteroidal
 [T.sub.4] to thyroid-binding    anti-inflammatory drugs, furose-
 globulin                        mide, heparin (?)
Cause increased thyroid-         Estrogens, tamoxifen, methadone,
 binding globulin                heroin, 5-fluorouracil, clofibrate,
                                 perphenazine, mitotane
Reduce thyroid-binding           Androgens, glucocorticoids,
 globulin                        aspariginase
Influence absorption             Cholestyramine, aluminum
 of thyroxine                    hydroxide, ferrous sulfate, sucral-
                                 fate, cation exchange resins

Acknowledgments. The authors gratefully acknowledge the assistance of Patsy Ellis and Nancy Milligan of the Mountain Home Veterans Administration Medical Center Library Service and the secretarial assistance of Dolores Moore and Ernestine Stewart at East Tennessee State University.

References

(1.) Ladenson PW, Singer PA, Ain KB, et al: American Thyroid Association guidelines for detection of thyroid dysfunction. Arch Intern Med 2000, 160:1573-1575

(2.) Stockigt JR: Hyperthyroxinemia secondary to drugs and acute illness. Endocrinologist 1993; 3:67

(3.) DeGroot LJ, Mayor G: Admission screening by thyroid function tests in an acute general care teaching hospital. Am J Med 1992;93:558

(4.) Braverman LE, Utiger RD (eds): Werner and Ingbar's The Thyroid: A Fundamental and Clinical Text. Philadelpia, Pa, Lippincott-Raven, 7th Ed, 1996, pp 387-393

(5.) Stockigt JR: Free thyroid hormone measurement, A critical appraisal. Endocrinol Metab Clin North Am 2001; 30:265-289

(6.) Figge J, Leinung M, Goodman AD: The clinical evaluation of patients with subclinical hyperthyroidism and free triiodothyronine (free [T.sub.3]) toxicosis. Am J Med 1994;96:229-234

(7.) Harjai KJ, Licata AA: Effects of amiodarone on thyroid function. Ann Intern Med 1997; 126:63-73

(8.) Amino N, Yabu Y, Miki T, et al: Serum ratio of triiodothyronine to thyroxine, and thyroxine-binding globulin and calcitonin concentrations in Graves' disease and destruction-induced thyrotoxicosis. J Clin Endocrinol Metab 1981; 53:113-116

(9.) Takamatsu J, Kuma K, Mozai T: Serum triiodothyronine to thyroxine ratio: a newly recognized predictor of the outcome of hyperthyroidism due to Graves' disease. J Clin Endocrinol Metab 1986; 62:980-983

(10.) DeGroot LJ, Larsen PR, Henneman G, et al (eds): The Thyroid and Its Diseases. New York, Churchill-Livingstone, 6th Ed, 1996

(11.) Gerding MN, van der Meer JW, Broenink M, et al: Association of thyrotropin receptor antibodies with the clinical features of Graves' ophthalmopathy. Clin Endocrinol (Oxf) 2000; 52:267-271

(12.) Khoo DH, Ho SC, Seah LL, et al: The combination of absent thyroid peroxidase antibodies and high thyroid-stimulating immunoglobulin levels in Graves' disease identifies a group at markedly increased risk of ophthalmopathy. Thyroid 1999; 9:1175-1180

(13.) Ross DS: Serum thyroid-stimulating hormone measurement for assessment of thyroid function and disease. Endocrinol Metab Clin North Am 2001; 30:245-264, vii

(14.) Winter WE, Signorino MR: Review: molecular thyroidology. Ann Clin Lab Sci 2001; 31:221-244

(15.) Abel ED, Ahima RS, Boers ME, et al: critical role for thyroid hormone receptor [[beta].sub.2] in the regulation of paraventricular thyrotropin-releasing hormone neurons. J Clin Invest 2001; 107:1017-1023

From the Mountain Home Veterans Affairs Medical Center and the Department of Medicine, East Tennessee State University Johnson City.

Reprint requests to Alan N. Peiris, MD PhD, Department of Medicine, East Tennessee State University, PO Box 70622, Johnson City, TN 37614.