Tests
1st tests to order
thyroid-stimulating hormone (TSH)
Test
Recommended screening test to assess thyroid function.
Do not order more laboratory tests until TSH is confirmed as abnormal.[22]
Result
low TSH suggests thyrotoxicosis; elevated in hypothyroid phase
serum free T4 and T3
Test
Confirms overt (or subclinical) thyrotoxicosis and measures the degree of thyrotoxicosis.
Result
elevated in the thyrotoxic phase (or within the upper normal range if thyrotoxicosis is minimal); low in the hypothyroid phase
TPO (thyroid peroxidase) antibodies
TSH-receptor antibodies (TRAb)
Test
Helps to distinguish thyroiditis from Graves disease. Can be used during the thyrotoxic phase. A positive result suggests Graves disease.
Uses an immunoassay (thyroid-stimulating immunoglobulins [TSI] or thyrotropin-binding inhibitory immunoglobulin [TBII]). TSI is highly specific (but not easily available), while TBII also measures blocking antibodies.
Result
negative
4-, 6-, or 24-hour radioiodine uptake
total T3/T4 ratio
Test
Helps to distinguish thyroiditis from Graves disease and toxic nodular goiter when the radioiodine uptake is contraindicated (e.g., breast-feeding postpartum).[23]
Result
<20 (nanograms/dL)/(micrograms/dL)
Tests to consider
technetium-99m pertechnetate scan
Test
Alternative to radioiodine uptake if unavailable, but may give equivocal results.
Result
absent or minimal concentration of technetium-99m pertechnetate into thyroid tissue
serum thyroglobulin
Test
Ordered only when factitious ingestion of thyroid hormone is a differential consideration.
Result
elevated (but the presence of antithyroglobulin antibodies may yield false-negative results)
thyroid biopsy
Test
Conducted in thyrotoxic phase, but seldom necessary.
Result
lymphocytic infiltrate
thyroid ultrasound
Test
Color-flow doppler ultrasound is another alternative to radioiodine uptake but requires extensive familiarity with the technique (widely available but uncommonly used).[25] Do not order thyroid ultrasound as part of the initial investigations for hyperthyroidism if there is no palpable abnormality of the thyroid gland.[26][27] Incidentally discovered benign thyroid nodules are common. Routine ordering of thyroid ultrasound will frequently identify nodules that are unrelated to the abnormal thyroid function. This may divert the clinical evaluation to assess the nodules, rather than the thyroid dysfunction.[28]
Result
reduced flow
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