Because the palpable abnormality had decreased uptake on thyroid scintigraphy, the nodule was biopsied using fine needle aspiration, and a diagnosis of papillary carcinoma was made. The patient then underwent subtotal thyroidectomy and the diagnosis of carcinoma was confirmed. Lymph node sampling revealed no evidence of distant tumor. Future plans include a metastatic survey with I-131.
In general, work-up of a nodule may include the following general categories:
Features in the history which increase the likelihood that a given nodule represents carcinoma include:
A variety of imaging modalities can be used to image thyroid nodules including thyroid scintigraphy, ultrasound, CT, and MRI. Practically speaking, scintigraphy and ultrasound have been most widely used to help differentiate benign versus malignant disease.
The three agents commonly used in thyroid scintigraphy include Tc-99m, I-123, and I-131. I-123 is often used because it is physiologic (both transported and organified) and it gives a reasonably low total body dose. However, I-123 exams usually require that the patient returns 24 hours following the administration of the tracer making this a two day procedure. Radiation dose with Tc-99m is also reasonably low and there is the advantage that images can be obtained the same day. However, there is the occasional problem of discordant nodules when using Tc-99m (tumors may be hot on Tc-99m, cold on I-123). The possibility of discordant nodules arises because Tc-99m is only transported and not organified, and some tumors can transport Tc-99m. I-131 is not used for normal thyroid imaging because of the high radiation burden and poor imaging characteristics. Types of nodules and the most common I-123 imaging findings are given below:
| Type of Nodule | I-123 features |
|---|---|
| Functioning Adenoma | Increased |
| Non-functioning Adenoma | Decreased |
| Multinodular goiter | Increased and Decreased |
| Colloid Nodule | Decreased |
| Cyst | Decreased |
| Malignant Tumor | Decreased |
| Local Thyroiditis | Increased or Decreased |
Scintigraphic findings in thyroid nodules are non-specific. Focal areas of decreased uptake are often called cold nodules. These cold regions are of concern because they can potentially represent malignant disease; however, the likelihood of carcinoma for any given cold nodule is generally considered to be less than 20% with more recent literature showing an incidence of only 4% (1). On the other hand, finding multiple cold areas interspersed between regions of increased activity can indicate a multinodular gland where there is a low incidence of associated malignancy.
A focal area of increased uptake is called a hot nodule. These hot nodules virtually never represent malignant disease but instead usually represent either autonomous or hypertrophic adenomas. Thyroid function tests and suppression scans can play a role in working up these hot lesions.
As with scintigraphy, ultrasound findings in the work-up of nodules are often non-specific. Occasionally, a simple cyst can be found and in this situation no further work-up is usually required. However, simple cysts are rare and any nodule found usually requires more investigation. Some clinicians use ultrasound to guide biopsies or to follow the size of nodules.
Fine needle aspiration (FNA) is widely regarded as the procedure of choice in evaluating nodules. Numerous studies since the 1970’s have shown the usefulness of this procedure. Reported sensitivities in detecting carcinoma by FNA range from 65%-99% with specificities ranging from 72%-100% (2). As might be expected, experience plays a critical role in attaining the highest sensitivities and specificities. The use of FNA has reduced the numbers of thyroidectomies by 25% and has increased the yield of carcinoma at surgery to 30% from 15%. Problems with FNA include difficulty differentiating follicular adenoma from carcinoma, false positives in Hashimoto’s thyroiditis, and false negatives due to inadequate sampling.
The work-up of thyroid nodules remains somewhat controversial. A good history, physical exam, and thyroid function tests often provide valuable clues in reaching a correct diagnosis but are non-specific. Despite difficulties with FNA, it is usually the initial procedure of choice. However, thyroid scintigraphy and ultrasound can both play useful roles in the evaluation of a nodule. Scintigraphy can be particularly useful in diagnosing a hot nodule where the risk of malignancy is extremely low. US can be used to guide biopsies, follow the size of a nodule, and differentiate cystic from solid lesions.
2. Mazzaferri EL. Management of a solitary thyroid nodule. NEJM 1993;8:553-9
3. Society of Nuclear Medicine. Procedure Guideline for Thyroid Scintigraphy. Version 1. June 24, 96.
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