Thyroid Scan and Uptake
Thyroid scintigraphy and radioiodine uptake measure thyroid function and map the distribution of function in the gland. They are used to find the cause of thyrotoxicosis, to characterise nodules, and to measure the uptake that sets the dose for radioiodine treatment.
The scan uses technetium-99m pertechnetate (75 to 200 MBq intravenously, with images at 15 to 20 minutes), which is trapped by the thyroid but not organified.
Iodine-123 (3.7 to 7.4 MBq by mouth) is trapped and organified, and gives images at 4 to 6 hours with uptake at 4 to 6 and 24 hours.
Iodine-131 (a few MBq) is used for uptake only, with a dose that is higher for whole body imaging in thyroid cancer.
A gamma camera with a pinhole or parallel hole collimator gives images of the neck in anterior and oblique views, with a marker at the sternal notch. The scan takes 20 to 30 minutes.
Uptake is measured with a probe or camera, and expressed as the percentage of the dose in the gland.
The effective dose for technetium is about 1 to 4 mSv, and for iodine-123 about 1 to 2 mSv.
Thyrotoxicosis affects about 1 to 2% of people, with Graves disease in 60 to 80%. Thyroid nodules are palpable in 5% and found on ultrasound in up to 50 to 60% of adults, with 5 to 10% malignant.
Scintigraphy has a high accuracy to separate hyperfunctioning (almost always benign) from non-functioning nodules, and uptake is raised in Graves disease and low in thyroiditis.
Indications.
- Thyrotoxicosis with an unclear cause: Graves disease, toxic nodular goitre, toxic adenoma, thyroiditis, drug induced or factitious thyrotoxicosis.
- Solitary or dominant thyroid nodule with low thyroid stimulating hormone (to find a hot nodule) and before radioiodine treatment.
- Measurement of uptake before radioiodine treatment (dose calculation), and sometimes to separate causes of thyrotoxicosis.
- Congenital hypothyroidism: dysgenesis, ectopic thyroid, or dyshormonogenesis.
- Neck or mediastinal masses: retrosternal goitre, ectopic and lingual thyroid, and thyroglossal remnants.
- After thyroid surgery: remnant assessment before radioiodine ablation (with whole body iodine-131 or iodine-123 scanning).
- Thyroid cancer follow up: whole body iodine scanning for metastases (separate technique).
- Neck mass: confirmation of thyroid origin.
Not for routine evaluation of thyroid nodules with a normal thyroid stimulating hormone (ultrasound and fine needle aspiration are first), and not in the diagnosis of hypothyroidism.
Contraindications and precautions.
- Pregnancy: absolute contraindication to radioiodine. Technetium is used only if essential and the benefit is clear.
- Breast feeding: iodine-123 and iodine-131 are avoided, and feeding is interrupted for at least 12 hours for technetium (and longer for iodine-123), or stopped for iodine-131.
- Recent iodinated contrast (4 to 8 weeks), amiodarone (months), iodine containing supplements, kelp, antithyroid drugs (3 to 7 days for propylthiouracil and methimazole), thyroid hormone replacement (3 to 4 weeks for levothyroxine, 2 weeks for liothyronine) and iodine rich food affect uptake. Technetium scans are less affected.
- Low iodine diet for 1 to 2 weeks for iodine studies.
- Children: dose by weight, with caution.
- Allergy is rare.
Preparation and technique.
The patient stops the interfering drugs and follows a low iodine diet if iodine is used. Fasting is not needed for technetium, and is helpful for oral iodine (fast 2 hours before and 1 hour after the capsule).
For technetium, the injection is given with the patient seated, and images are taken at 15 to 20 minutes with the camera 5 to 10 cm from the neck. Markers show the sternal notch and any palpable nodule. A 24-hour image and uptake can be added with iodine.
Uptake is measured with a calibrated probe at 4 to 6 and 24 hours after iodine, or from the camera counts at 20 minutes for technetium. The dose to the gland is estimated for radioiodine treatment from uptake and gland size (ultrasound).
