Assays for FT4, total triiodothyronine (T3), and free triiodothyronine (FT3) have largely supplanted measurements of total T4, resin T3 uptake (RT3U), and free thyroxine index (FT4I). It is particularly important to determine "free" serum levels (FT4 and FT3) in conditions associated with high circulating levels of thyroxine-binding globulin (TBG), such as during therapy with oral estrogen. Ultrasensitive assays for serum TSH have largely replaced older TSH assays. Table 26–2 shows the appropriate use of thyroid tests.
Table 26–2.Appropriate use of thyroid tests. ||Download (.pdf) Table 26–2. Appropriate use of thyroid tests.
| ||Test ||Comment |
|For screening ||Serum thyroid-stimulating hormone (TSH) ||Most sensitive test for primary hypothyroidism and hyperthyroidism |
| ||Free thyroxine (FT4) ||Excellent test |
|For hypothyroidism ||Serum TSH ||High in primary and low in secondary hypothyroidism |
| ||Thyroid peroxidase and thyroglobulin antibodies ||Elevated in autoimmune (Hashimoto) thyroiditis |
|For hyperthyroidism ||Serum TSH ||Suppressed except in TSH-secreting pituitary tumor or pituitary hyperplasia (rare) |
| ||Triiodothyronine (T3) or free triiodothyronine (FT3) ||Elevated |
| ||123I uptake and scan ||Increased uptake; diffuse versus “hot” foci on scan |
| ||Thyroid peroxidase and thyroglobulin antibodies ||Elevated in Graves disease |
| ||Thyroid-stimulating immunoglobulin (TSI) ||Usually (65%) positive in Graves disease |
|For thyroid nodules ||Fine-needle aspiration (FNA) biopsy ||Best diagnostic method for thyroid cancer |
| ||123I uptake and scan ||Cancer is usually “cold”; less reliable than FNA biopsy |
| ||99mTc scan ||Vascular versus less vascular |
| ||Ultrasonography ||Useful to assist FNA biopsy. Useful in assessing the risk of malignancy (multinodular goiter or pure cysts are less likely to be malignant). Useful to monitor nodules and patients after thyroid surgery for carcinoma. |