Pathology/Lab Coding Alert

"Physicians often order other thyroid laboratory tests based on abnormal TSH and FT4 levels that indicate hypo- or hyperthyroidism," Liles says. The determination of medical necessity for many of these other tests depends on the diagnosis of thyroid dysfunction, rather than on the signs and symptoms that prompted the initial TSH and FT4 tests.

For example, low TT4 with elevated TSH might indicate primary hypothyroidism, and the physician may order a confirmatory test such as anti-thyroperoxidase antibody (TPO-Ab) using 86376 (Microsomal antibodies [e.g., thyroid or liver-kidney], each) to determine if the cause is an autoimmune disorder such as Hashimoto's thyroiditis.

Normal TSH with low FT4 might indicate, for example, secondary hypothyroidism due to pituitary dysfunction. To further define this diagnosis, a physician may order an evocative/suppression test such as 80438 (Thyrotropin releasing hormone [TRH] stimulation panel; one hour) or CPT 80439 ( two hour).

Hyperthyroidism, on the other hand, is often indicated by elevated FT4 and depressed levels of TSH. The TPO-Ab test (86376) and TRH stimulation panels (80438-80439) may also be useful in diagnosing autoimmune forms of this dysfunction, such as Graves' disease, along with a test for thyroid stimulating immune globulins (TSI) (84445), now often referred to as TSH receptor antibodies. Dettwyler says a diagnosis of hyperthyroidism may indicate medical necessity for other confirmatory tests such as thyrotropin receptor antibody (83519, Immunoassay, analyte, quantitative; by radiopharmaceutical technique [e.g., RIA]) and free triiodothyronine (84481, Triiodothyronine T3; free), which is useful in distinguishing various forms of hyperthy-roidism and monitoring patients on antithyroid therapy. For symptomatic patients with normal FT4 levels, physicians may order additional tests for total and free T3 (84480, Triiodothyronine T3; total; and 84481, free), which are instrumental in the diagnosis of T3 thyrotoxicosis.

Patients with severe or chronic illness or multiple medications may exhibit many abnormalities of thyroid hormone balance, known as euthyroid sick syndrome. Because results of TSH, T3 and FT4 testing may be variable for these patients, the physician may order TT4 (84436), which may provide more diagnostic accuracy in these cases. Similarly, the physician may order reverse T3 (84482, Triiodothyronine T3; reverse) to help distinguish euthyroid sickness from true hypo- or hyperthyroidism.

Physicians might order other laboratory tests if they suspect thyroid cancer. For example, labs might evaluate serum levels of thyroglobulin, a T4 production protein that serves as a tumor marker for thyroid carcinoma using 84432 (Thyro-globulin). The physician may also order a thyroglobulin antibody test to monitor patients with diagnosed thyroid cancer, reported as 86800 (Thyroglobulin antibody).