Investigations
1st investigations to order
serum thyroid-stimulating hormone (TSH)
serum total T4, total T3, free T3, free T4 index, and free T4
T3:T4 ratio
Test
In the moderately or highly thyrotoxic subacute thyroiditis patient, the T3:T4 ratio is generally <16 if assessed by total T3:total T4, or <3.0 if assessed by free T3:free T4.[39][40][41]
Values should be interpreted with caution; clinical judgement is required. Additional diagnostic studies may be necessary for definitive diagnosis.
Result
total T3:total T4 ratio <16; free T3:free T4 ratio <3
FBC
Test
Mild anaemia and elevation of white blood cell count are common.[4][28] JTA: guideline for the diagnosis of subacute thyroiditis (acute phase) Opens in new window
Result
may show low level of haemoglobin or haematocrit; leukocyte count may be raised
serum CRP
Test
A non-specific marker of inflammation. Significantly raised (i.e., >10 mg/L) in 86% of patients with subacute thyroiditis in one study.[42]
May help to differentiate subacute hypothyroidism from Graves’ disease.[42][43]
Thus serum CRP levels may help when, based on laboratory and imaging studies, the diagnosis of subacute thyroiditis is not clear.[44]
Result
raised
serum erythrocyte sedimentation rate (ESR)
Test
A non-specific marker of inflammation. Likely to be raised in most patients.
The average ESR was 53 mm/hour in one study of patients with subacute thyroiditis.[2]
Result
raised
radioactive iodine uptake (RAIU)
Test
In the thyrotoxic phase, scintigraphy demonstrates low thyroidal uptake (I-123 or 99mTc-pertechnetate), typically <1% to 3% at 24 hours.[4][9][Figure caption and citation for the preceding image starts]: I-123 radioactive iodine scan showing absence of thyroid uptake in the thyrotoxic phase of subacute thyroiditis; arrow indicates sternal notch markerFrom the personal collection of Dr Stephanie Lee [Citation ends].
Reliably confirms the diagnosis of subacute hypothyroiditis in the thyrotoxic phase. RAIU can be obtained if the clinical presentation and serum thyroid function tests are not sufficient for diagnosis.
May be raised or normal during the recovery from hypothyroidism.[1][2]
Result
very low thyroidal uptake during the thyrotoxic phase, typically <1% to 3% at 24 hours.
Investigations to consider
fine needle aspiration biopsy
Test
Biopsy is not routinely performed.[4]
Cytology (fine needle aspiration biopsy) may be useful to confirm a clinical diagnosis in the setting of: high iodine intake (e.g., a patient who recently received iodinated contrast for a radiological scan [within the past 2 months, approximately]); recent use of an iodine-rich drug (e.g., amiodarone [within the past 6 months, approximately).
Saturation of the thyroid's capacity to absorb radioactive iodine in these scenarios can lead to falsely low uptake.
Result
cytological features consistent with subacute thyroiditis include multinucleated giant cells, degenerated follicular epithelium cells, epithelioid granulomas, and mixed inflammatory cells
ultrasonography
Test
Ultrasound is not sufficiently specific to confirm the diagnosis of subacute thyroiditis; findings can be similar to the sonographical appearance of chronic thyroiditis or suspicious thyroid nodules.[34] Ultrasound should not, therefore, be used alone for the diagnosis of subacute thyroiditis.
Ultrasound elastography demonstrates that subacute thyroiditis lesions may have an elevated baseline elasticity score compared with benign nodules of a multinodular goitre or chronic autoimmune thyroiditis.[35][36]
However, this imaging modality was unable to distinguish between subacute thyroiditis and thyroid cancer in one study.[35]
Result
patients with painful subacute thyroiditis have areas of poorly-defined heterogenous hypoechoic echotexture, with irregular margins in the areas of the thyroid gland that are painful; normal or decreased flow may be evident on colour Doppler ultrasound
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