From the Guidelines
A comprehensive thyroid function workup should begin with a thyroid-stimulating hormone (TSH) measurement, and if TSH is abnormal, free T4 and, if indicated, total T3 can be measured, as recommended by the most recent and highest quality study 1. When evaluating thyroid function, it is essential to consider the most recent guidelines and evidence-based practices.
- The initial screening test for thyroid dysfunction is the serum TSH test, which can help identify most common thyroid disorders, including hypothyroidism and hyperthyroidism 1.
- If the TSH level is abnormal, follow-up testing of serum T4 levels can differentiate between subclinical and overt thyroid dysfunction 1.
- Additional testing, such as free triiodothyronine (free T3), may be necessary in cases of suspected hyperthyroidism where T3 toxicosis may occur with normal T4 levels.
- Thyroid antibody tests, including thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), can help evaluate for autoimmune thyroid diseases like Hashimoto's thyroiditis, and thyroid stimulating immunoglobulins (TSI) or TSH receptor antibodies (TRAb) for Graves' disease.
- In cases where thyroid nodules or cancer are suspected, thyroglobulin levels may be ordered, and reverse T3 can occasionally be helpful in complex cases to distinguish between true hypothyroidism and euthyroid sick syndrome. The most recent study 1 provides the highest quality evidence for guiding thyroid function workups, and its recommendations should be prioritized in clinical practice.
- The optimal screening interval for thyroid dysfunction is unknown, but the USPSTF recommends screening with a TSH test and confirming or ruling out abnormal findings with multiple tests over a 3- to 6-month interval 1.
- Treatment for hypothyroidism typically involves oral T4 monotherapy (levothyroxine sodium), while hyperthyroidism is treated with antithyroid medications or nonreversible thyroid ablation therapy 1.
From the Research
Laboratory Tests for Thyroid Function Workup
The following laboratory tests are recommended for a thyroid function workup (Thyroid Function Tests, TFTs):
- Serum hormone concentrations, such as TSH, FT4, and FT3, to assess thyroid function 2
- Measurement of TSH, which is the most reliable and informative regarding thyroid function, to correct indicate a hyperthyroid, euthyroid, or hypothyroid state 2
- FT4, which supersedes total T4 (TT4), as TT4 is affected by changes in serum thyroid hormone binding proteins (TBG, TTR, Albumin) 2
- Detection of serum autoantibodies against thyroid tissues, such as TgAb, TPOAb, and TSH receptor antibody, to differentiate Graves' disease from Hashimoto's thyroiditis 2
- Measurement of free triiodothyronine (FT3) and TSH levels to further enhance test sensitivity and distinguish primary from secondary thyroid dysfunction 3
- Simultaneous detection of FT4 and TSH to evaluate real thyroid function, especially in patients with autoantibodies against thyroid hormone (THAb) 4
Interpretation of Laboratory Results
The interpretation of laboratory results is crucial in the diagnosis and management of thyroid disorders. The following should be considered:
- Reference intervals for normal healthy subjects of TSH are around 0.4-5.0 microU/ml 2
- The TSH level, together with FT4 level, should be determined to allow the differentiation and assessment of the intensity of thyroid function disorders and foresee its consequences 5
- The interpretation of TSH results in screening tests is different in certain population groups, such as children aged under 14, pregnant women, the elderly, and patients with non-thyroidal illnesses 5