The scan assesses the function of the thyroid gland and of thyroid nodules.
When should I order a thyroid scan?
1. Patients with hyperthyroidism.
Flow chart for the investigation of a patient with thyrotoxicosis
2. Patients with thyroid nodules.
What do I tell my patient? Allow 60 minutes.
No preparation required.
Patient is given an intravenous injection of a tracer and imaged 20 minutes later.
Normal Thyroid Scan
What will the scan tell me?
1. Patients with hyperthyroidism
There are four major causes for hyperthyroidism: Graves’ disease, toxic multinodular goitre, autonomous functioning thyroid nodule and thyroiditis. The thyroid scan can easily differentiate these conditions. In Graves’ disease, the scan demonstrates diffusely increased uptake; in toxic multinodular goitre, there are hyperfunctioning areas and hypofunctioning areas throughout the thyroid; in autonomous functioning thyroid nodule, there is a solitary hyperfunctioning area with suppression of uptake of the rest of the thyroid; and in thyroiditis, there is minimal uptake in the thyroid.
2. Patients with thyroid nodules
Thyroid nodules are very common with 50% of people over fifty years having thyroid nodules on ultrasound. The great majority of these nodules are not malignant. Thyroid nodules under 1 cm in size do not need to be investigated with thyroid scans, ultrasounds or fine needle aspiration biopsy (FNAB). There is a very low chance that such nodules are malignant and these can be watched clinically or with follow up ultrasound to detect if the nodule is growing. Provided that the patient is not thyrotoxic, the best investigation of a large thyroid nodule is a FNAB without any thyroid scan or ultrasound. In this setting, most thyroid nodules are hypofunctioning on thyroid scans and there are no ultrasound findings that can exclude malignancy. While FNAB can be performed without ultrasound guidance, there is good evidence that the accuracy of this technique is improved when the FNAB is coupled with ultrasound. Prior to ultrasound guided FNAB, the size of the nodule can be measured accurately which may help in future management of the patient. Unfortunately, FNAB’s are far from perfect with about 20% of FNAB’s being non diagnostic and may miss malignant nodules in up to 5% of patients.
Hypofunctioning or cold nodule in lower pole of
right lobe of thyroid gland.
What next?
In hyperthyroid patients, the diagnosis is made and appropriate treatment is instituted.
In patients with a large hypofunctioning or cold nodule, fine needle aspiration biopsy
is usually indicated.
What is a parathyroid scan?
The scan localises parathyroid adenomas.
When should I order a parathyroid scan?
1. Patients with primary hyperparathyroidsm
What do I tell my patient?
Allow 3 hours.
No preparation required.
The patient will receive an intravenous injection of a tracer and will be scanned
15 minutes later and then again in two hours time. There are no restrictions on
the patient during the two hour break.
What will the scan tell me? The scan will localise the site of the parathyroid adenoma. A negative scan can be
seen in cases of parathyroid hyperplasia.
Parathyroid scan of a 65 year old patient with primary hyperparathroidism. The initial scan on the left shows a small area of abnormal uptake adjacent to the inferior pole of the left lobe of thyroid. Delayed image on right performed 2 hours later demonstrates persisting uptake in this area typical of a parathyroid adenoma.
What next? Positive scans confirm the diagnosis and assist in surgical planning. In the case of negative scan, a thyroid scan is often performed to ensure that a small adenoma is not missed.
