Thyroid Scintigraphy and RAIU Findings in Hyperthyroidism with Thyroid Nodule
In this patient with hyperthyroidism and a solitary thyroid nodule, the thyroid scintigraphy and radioactive iodine uptake scan will most likely show increased uptake only in the nodular tissue (option E). 1
Clinical Assessment and Diagnosis
This 48-year-old female presents with classic symptoms of hyperthyroidism:
- Excessive sweating
- Heat intolerance
- Loose stools
- Insomnia
- Palpitations
- Tachycardia (HR 121)
- Low TSH (0.01 mIU/mL)
- Elevated T4 (13.2 ng/dL)
- Palpable thyroid nodule in the left lobe
- Bounding pulse and resting tremor
These findings strongly suggest a toxic thyroid nodule (toxic adenoma), which is an autonomously functioning thyroid nodule producing excess thyroid hormone independent of TSH regulation.
Imaging Findings in Toxic Adenoma
In a toxic adenoma:
- The nodule produces thyroid hormone autonomously (independent of TSH)
- The high thyroid hormone levels suppress TSH
- The suppressed TSH causes the normal thyroid tissue to become inactive
- On scintigraphy, this creates the characteristic pattern of:
- Increased uptake in the nodule ("hot nodule")
- Decreased or absent uptake in the surrounding normal thyroid tissue
According to the American College of Radiology guidelines, radionuclide uptake and scan help differentiate between causes of thyrotoxicosis 1. In toxic adenoma, the scan will show a focal area of increased uptake corresponding to the nodule, with suppression of the surrounding normal thyroid tissue.
Differential Diagnosis Based on Imaging Patterns
| Pattern | Diagnosis | Description |
|---|---|---|
| Increased uptake only in nodule | Toxic adenoma | Single hyperfunctioning nodule with suppressed surrounding tissue |
| Diffuse increased uptake | Graves' disease | Homogeneous increased uptake throughout the gland |
| Multiple areas of increased uptake | Toxic multinodular goiter | Multiple hyperfunctioning nodules |
| Very low uptake throughout | Thyroiditis | Inflammation causing release of preformed hormone |
| Normal uptake | Not typical in hyperthyroidism | Would not explain clinical picture |
Why Other Options Are Incorrect
- Option A (entire thyroid "hot" with diffuse uptake): This pattern is characteristic of Graves' disease, not a solitary toxic nodule 1, 2
- Option B (increased uptake in non-nodular tissue only): This pattern doesn't occur in any common thyroid disorder
- Option C (entire thyroid "cold" without uptake): This pattern suggests thyroiditis, which typically presents with transient hyperthyroidism but would not explain the persistent nodule 3, 2
- Option D (normal RAIU scan): A normal scan would not explain the clinical hyperthyroidism 4
Clinical Implications
The finding of a toxic adenoma on scintigraphy has important management implications:
- Confirms the diagnosis of toxic adenoma as the cause of hyperthyroidism
- Guides treatment options (radioiodine ablation, surgery, or antithyroid medications)
- Indicates that the nodule is very likely benign (hot nodules are rarely malignant)
- Suggests that fine needle aspiration biopsy is not necessary for this nodule
Common Pitfalls to Avoid
- Failing to recognize that a palpable nodule in a patient with hyperthyroidism suggests a toxic adenoma rather than Graves' disease
- Misinterpreting low radioiodine uptake as thyroiditis when clinical features suggest a toxic nodule
- Assuming all thyroid nodules need biopsy (hot nodules on scintigraphy rarely harbor malignancy)
- Not correlating laboratory findings with imaging results (low TSH with a nodule strongly suggests toxic adenoma)
In conclusion, based on the clinical presentation of hyperthyroidism with a palpable thyroid nodule, low TSH, and elevated T4, the most likely finding on thyroid scintigraphy and RAIU scan would be increased uptake only in the nodular tissue (option E).