Thyroid scan
From Radipedia
Uptake and Scan
I123
- Taken up and oranified
- Oral dose 200-600 uCi (1.5 mCi for thyroid cancer scans)
- Normal 4 hour uptake is 15%
- Normal 24 hour uptake is 30%
- No background uptake differentiates from Tc99m
Tc99m pertechnetate
- Can also be used but not organified
- Lower patient dose
Differential
Graves Disease - aka diffuse toxic goiter, autoimmune disease caused by autoantibodies to the TSH receptor (aka Thyroid Stimulating Immunoglobulin TSI).
Toxic nodular goiter - Plummer's disease, functionally autonomous nodules, emerges insidiously from multinodular goiter.
Thyroiditis
- Chronic (Hasimoto's) - may mimic graves early on and multinodular goiter later, the thyroid gland is gradually destroyed by a variety of cell and antibody mediated immune processes.
- Acute Bacterial or Subacute Viral - more of a clinical diagnosis, decreased uptake (less than 1% at 24hr), elevated T4 (clinical hyperthyroid).
Cold Nodule - most commonly a colloid cyst, but may represent neoplasm, correlate with US
Hot Nodule - most are benign autonomous adenomas
Multinodular Goiter - enlarged gland with multiple hot and cold nodule giving a patchy appearance. These cold nodules do not have the same risk of cancer as a single cold nodule.
Decreased Uptake
- Antithyroid drugs (I123 only bc organified)
- Iodine load (CT contrast)
- Subacute thyroiditis (viral and painful)
- Silent thyroiditis (non-tender)
