The term thyroiditis covers several conditions, the most common being autoimmune (Hashimoto’s) thyroiditis. Thyroid scintigraphy is often superfluous in thyroiditis, with a few exceptions as discussed next. Autoimmune Thyroiditis Autoimmune thyroiditis may give rise to thyrotoxicosis due to ex acerbation of the autoimmune inflammation, which may be clinically apparent as silent thyroiditis and postpartum thyroiditis. In particular, it is important to differentiate between postpartum thyroiditis and Graves’ disease. In postpartum thyroiditis, an inflammatory destructive condition with no active thyroid hormone synthesis, thyroid scintigraphy shows universally very low or no iso tope uptake. However, the distinction between the two conditions can often be made without thyroid imaging. Nearly all patients with Graves’ disease harbour TSH- receptor antibodies, which are absent in typical cases of postpartum thyroiditis. In addition, the dynamics of the two diseases are very different, as the thyrotoxic phase in post-partum thyroiditis is followed by hypothyroidism, before recovery of the thyroid function takes place within some months. With such a course, the diagnosis is usually settled, and thyroid scintigraphy is of no value at this late phase of the disease. Importantly, radioactive isotopes should not be administered to a breastfeeding woman. If necessary, breast milking in advance (with storage in the refrigerator), and no breastfeeding during 24– 48 hours after the scintigraphy complies with the safety regulations.
Subacute Thyroiditis (de Quervain’s Thyroiditis)
The course of the thyroid hormone serum levels in subacute thyroiditis is similar to that seen in silent or postpartum thyroiditis. Accordingly, thyroid scintigraphy initially shows a very low or no thyroid uptake. Subacute thyroiditis may affect the gland unilaterally, with initially no isotope uptake in a single thyroid lobe, and with subsequent progression to the entire gland within weeks.
Occasionally, typical symptoms like neck pain may be absent in subacute thyroiditis. In the undiagnosed patient, this may prompt an 18F- deoxy- glucose positron emission tomography (FDG- PET). If performed in the thyrotoxic phase, an increased FDG uptake in the thyroid and skeletal muscle is seen, while the uptake in the liver is decreased.
