Thyroid Case 2 Answers
1) How can one determine if thyroid uptake is increased, normal or decreased?
Thyroid uptake on pertechnetate scans can be assessed by quantitative or qualitative methods. An estimate of thyroid uptake can be obtained by comparing administered activity in the syringe prior to injection with activity in the thyroid at 20 minutes. The normal range is 0.4-4% of administered dose, but the normal range will vary between departments.
However, it is usually adequate to make a qualitative visual assessment of thyroid uptake by comparing levels of activity in thyroid and salivary glands. The normal thyroid will have similar or slightly higher activity than salivary glands. If activity is clearly greater or less than that in salivary glands then thyroid uptake can be taken to be increased or decreased, respectively.
2) Is uptake normal in this case?
No – activity is homogeneously increased throughout the thyroid gland which is also arguably mildly enlarged. There is relatively little activity in salivary glands but activity can still be visualised within the stomach on the unzoomed view.
3) What is A and what is its significance?
This represents the pyramidal lobe of the thyroid gland. It is an accessory process of thyroid tissue which can arise from the superior aspect of the isthmus (as in this case) or from either lobe. Although it is a normal anatomical variant, it is relatively unusual for it to be visible on a normal thyroid pertechnetate or I123 scan. If seen it implies there is increased thyroid uptake, and it is particularly associated with autoimmune hyperthyroidism (refs 1,2).
4) What is the likely diagnosis in this case?
In a patient with thyrotoxicosis, this pattern of homogeneously increased uptake throughout the thyroid gland, especially with a visible pyramidal lobe, is highly suggestive of Graves’ disease. This is an autoimmune condition most common in young adults with a strong female predominance (7:1). It is associated with the production of IgG antibodies directed towards the TSH receptor on thyroid follicular cells and which causes TSH-independent stimulation of this receptor resulting in excess thyroid hormone production. The antibody will be detectable in approximately 90% of patients.
The other condition which could cause diffusely increased thyroid uptake like this in a thyrotoxic patient is Hashimoto’s disease. This is also an autoimmune condition which causes chronic hypothyroidism and reduced thyroid uptake, but in the early acute phase can cause increased uptake and hyperthyroidism. It is, however, extremely unusual for patients to present for thyroid scintigraphy at this stage, and increased uptake is usually fairly modest.
5) Would it matter if the patient had commenced medical treatment for hyperthyroidism prior to the scan?
Pertechnetate is trapped by the thyroid active iodide transporter but is not organified. The use of thiocarbamide antithyroid drugs such as carbimazole and propylthiouracil does not interfere with its uptake. This is a major advantage of pertechnetate over I123 and I131 in this setting, the latter two agents having their uptake reduced by antithyroid drugs. Pertechnetate uptake will be reduced by anything which increases the blood iodide pool .
References
- Levy HA, Sziklas JJ, Rosenberg RJ, et al. Incidence of a pyramidal lobe on thyroid scans. Clin Nucl Med 1982; 7:560-1
- Wahl R, Muh U, Kallee E. Hyperthyroidism with or without pyramidal lobe Graves’ disease or disseminated autonomously functioning thyroid tissue? Clin Nucl Med 1997; 22:451-8
Contributed by Ian Hagan
The text is entirely the opinion of the author and does not necessarily reflect that of RUH NHS Trust or the Bristol Radiology Training Scheme. Website content devised by Paul McCoubrie.