Case Author(s): Feiyu Xue, M.D., Ph.D. and Tom R. Miller, M.D., Ph.D. , 6/22/04 . Rating: #D3, #Q3
Diagnosis: Counting error
48 year-old woman with nervousness, heat intolerance, and palpitation.
I-131 uptake calculated as 8.3%.
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View second image(mc).
Printout from the multichannel analyzer.
Full history/Diagnosis is available below
Diagnosis: Counting error
48 year old woman with nervousness, heat intolerance, and palpitation was found to have TSH < 0.01 uIU/ml and Free T4 4.3 ng/dl. She was referred to have I-131 uptake measurement and possible I-131 therapy. After a consult and physical examination, it was felt that the history, physical findings, and laboratory studies were consistent with Grave’s disease.
4.9 UCi I-131
The I-131 uptake was calculated as 8.3%, contrart to the clinical history, laboratory and physical findings. Inspection of the printout from the multichannel analyzer (see Image 2) showed the initial counting was performed for 63.6 seconds (giving 3650 counts) instead of the routine 300 seconds (5 minutes) used for the measurment of the I-131 standard. The energy spectrum was correct.
The differential diagnosis of low I-131 uptake in a patient with hyperthyroidism:
Hashimoto thyroiditis (early)
Anti-thyroid medications (Tapazole, PTU)(early after starting meds)
Recent Iodinated contrast administration
Ectopic thyroid tissue, Struma ovarii
Instrument failure/counting errors
In this patient with typical symptoms and signs of Grave’s disease, and no history of subacute thyroiditis, recent iodinated contrast use or medication with Aniodarone, Tapazole, or PTU, a counting error was suspected. Inspection of the multichannel analyzer print-out reveals the error in counting duration. Repeat count for 5 minutes showed I-131 uptake of 41% based on total patient count of 16880 for 5 min (Image 3). Patient was diagnosed with Grave’s disease and was treated with a therapeutic dose of I-131.
Repeat measurement for 300 seconds.
View followup image(mc).
Repeat counting for 5 minutes.
Major teaching point(s):
Technical error should always be the first thing to rule out when imaging or test data disagree with clinical findings.
Differential Diagnosis List
Subacute thyroiditis, Hashimoto thyroiditis (early), Anti-thyroid medications (Tapazole, PTU early after starting meds), Amiodarone, Recent iodinated contrast administration, Thyrotoxicosis Factitious, Ectopic thyroid tissue,, Struma ovarii , Instrument failure/counting errors
ACR Codes and Keywords:
References and General Discussion of Thyroid Scintigraphy (Anatomic field:Spine and Contents, Category:Other generalized systemic disorder)
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Case number: tr017
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