Subacute Thyroiditis with Autonomous Nodule
The Tc-99m pertechnetate scan shows poor
uptake in the thyroid. On the marker view, the thyroid gland has
very low uptake compared to the salivary gland activity. Hyperthyroidism
with a non-functioning thyroid gland is most often due to subacute thyroiditis.
The differential diagnosis includes exogenous administration of thyroid
hormone, and rarely ectopic thyroid hormone production.
The pertechnetate scan shows a small focus of tracer uptake in the right
upper pole, representing an autonomous nodule (a nodule not responsive
to TSH levels). These findings are similar to those seen with a toxic
autonomous nodule which suppresses tracer uptake in the remainder of the
thyroid. Toxic autonomous nodules, however, are larger and demonstrate
more intense tracer uptake than the small focus noted on this study.
The iodine-123 scan performed 2 months later shows homogenous uptake
of iodine in the gland. The 24 hour iodine uptake was elevated, 43%,
compatible with the recovery phase of subacute thyroiditis.
Uptake of Tc-99m pertechnetate depends only on the trapping function
of the thyroid gland. Uptake of iodine-123 depends both on trapping
and on organification. At times, Tc-99m pertechnetate and iodine-123
thyroid scans show disperate function in nodules. In this case, the
difference between the two scans is compatible with the natural history
of subacute thyroiditis.
A 50 year old woman with hyperthyroidism had an iodine-123 scan on 1/14/98
(shown on the top row; the view on the right shows a marker on the sternal
notch and a second marker 10 cm above the sternal notch). The scan
shows an autonomous nodule in the right lower pole. The 24 hour iodine
uptake was suppressed at 3%. Suppressed thyroid function with hyperthyroidism
is compatible with subacute thyroiditis. There is an autonomously
functioning nodule in the right lower neck (the position with respect to
the sternal notch marker ican be noted on the last image on the first row).
A repeat iodine-123 scan 4 months later showed uptake throughout the gland.
The 24 hour iodine uptake was slightly elevated at 37%, compatible with
recovering subacute thyroiditis. The nodule in the right lower pole
is slightly hypofunctioning compared to the rest of the gland.
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J. Anthony Parker, MD PhD, Tony_Parker@bidmc.harvard.edu