Investigations
1st investigations to order
thyroid-stimulating hormone (TSH)
Test
Recommended screening test to assess thyroid function.
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
thyroid peroxidase (TPO) 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 goitre when the radioiodine uptake is contraindicated (e.g., breastfeeding postnatally).[22]
Result
<0.024 in molar SI units [<20 (nanograms/dL)/(micrograms/dL) in conventional units]
Investigations 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 anti-thyroglobulin antibodies may yield false-negative results)
thyroid biopsy
Test
Conducted in thyrotoxic phase, but seldom necessary.
Result
lymphocytic infiltrate
colour-flow Doppler ultrasound
Test
Another alternative to radioiodine uptake but requires extensive familiarity with the technique (widely available but uncommonly used).[24]
Result
reduced flow
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