Graves' Disease is the Most Likely Cause
In a patient presenting with thyrotoxicosis and high radioactive iodine uptake (RAIU), Graves' disease is the most likely diagnosis, as it is the most common cause of hyperthyroidism and characteristically demonstrates elevated thyroid hormone production with increased iodine uptake. 1, 2, 3
Understanding the Diagnostic Pattern
The combination of thyrotoxicosis with high RAIU is the key distinguishing feature that narrows the differential diagnosis significantly 1:
High RAIU indicates active thyroid hormone synthesis within the gland itself, which differentiates true hyperthyroidism from destructive thyroiditis or exogenous hormone intake 1, 4
Low or absent RAIU would indicate destructive thyroiditis (such as subacute thyroiditis) or factitious thyrotoxicosis from exogenous thyroid hormone, making options A and D incorrect 1, 5
Why Each Answer Choice Fits or Doesn't
Graves' Disease (Correct Answer - C)
Graves' disease accounts for the majority of hyperthyroidism cases and characteristically shows RAIU greater than 30-35% at 24 hours 2, 6, 3
The scan demonstrates diffuse thyroid enlargement with intense, homogeneous tracer uptake throughout both lobes 2
This autoimmune condition is caused by thyroid-stimulating hormone receptor antibodies that continuously stimulate the gland 6, 3
Subacute Thyroiditis (Incorrect - A)
Subacute thyroiditis causes low or absent radioiodine uptake because it represents destructive inflammation of thyroid follicles with release of preformed hormone, not active synthesis 1
The American College of Radiology specifically states that low uptake indicates destructive thyroiditis, which is transient 1
While rare atypical cases with slightly elevated uptake have been reported, these are exceptional and would not be the "most likely" diagnosis 7
Struma Ovarii (Incorrect - B)
This ectopic thyroid tissue in ovarian teratomas can cause thyrotoxicosis, but uptake would be localized to the pelvis, not the thyroid gland 4
It represents an extremely rare cause of thyrotoxicosis and would not be the most likely diagnosis in a standard clinical presentation 6
Exogenous Thyroid Hormone (Incorrect - D)
Factitious thyrotoxicosis from surreptitious thyroid hormone ingestion shows very low thyroid uptake because exogenous hormone suppresses TSH and thyroid function 5
Diagnosis is supported by suppressed serum thyroglobulin levels and decreased RAI uptake 5
Clinical Algorithm for High RAIU Thyrotoxicosis
When RAIU is elevated, the pattern of uptake distinguishes the specific cause 4, 2:
- Diffuse, homogeneous uptake → Graves' disease 2
- Multiple patchy "hot" areas → Toxic multinodular goiter 4
- Single focal "hot" area with suppression elsewhere → Toxic adenoma 4
Key Diagnostic Pitfall to Avoid
Never assume the diagnosis without confirming TSH suppression and obtaining imaging 1, 4. The American College of Radiology emphasizes that:
- TSH should be measured first to confirm thyrotoxicosis 4, 2
- Thyroid ultrasound should be performed after suppressed TSH is documented to detect coexisting nodules that may require biopsy 2
- RAIU scan (preferably with I-123) follows to differentiate the underlying cause 1, 2
This systematic approach prevents missing concurrent thyroid malignancy and ensures appropriate treatment selection 2.