Quiz: Thyroid Scintigraphy and Nuclear Medicine — 26 questions

1. Which laboratory pattern most strongly indicates primary hyperthyroidism?

A. Decreased TSH with increased free T4 and free T3

B. Decreased TSH with decreased free T4 and free T3

C. Increased TSH with decreased free T4 and free T3

D. Increased TSH with increased free T4 and free T3

Answer: (A)

Explanation

Primary hyperthyroidism typically suppresses TSH while raising free T4 and free T3 through increased thyroid hormone production. The opposite pattern, elevated TSH with reduced free hormones, is characteristic of primary hypothyroidism.

2. Which combination correctly describes major functions of the thyroid gland?

A. It releases parathyroid hormone and calcitonin and controls ventilation and acid secretion

B. It secretes insulin and glucagon and controls blood glucose and calcium balance

C. It produces cortisol and aldosterone and regulates inflammation and sodium balance

D. It secretes T3 and T4 and helps regulate metabolism, growth, and thermogenesis

Answer: (D)

Explanation

The thyroid produces T3 and T4, which influence energy metabolism, growth, thermogenesis, and body weight. Insulin, cortisol, and parathyroid hormone are produced by other endocrine organs.

3. Which statement best distinguishes iodine-123 from iodine-131 in thyroid imaging and treatment?

A. Iodine-123 is mainly therapeutic, whereas iodine-131 is primarily diagnostic

B. Both isotopes are mainly therapeutic but differ in their uptake mechanisms

C. Iodine-123 is primarily diagnostic, whereas iodine-131 is mainly therapeutic

D. Both isotopes are primarily diagnostic but differ only in their administered volume

Answer: (C)

Explanation

Iodine-123 is used mainly for diagnostic thyroid imaging, while iodine-131 is used mainly for therapy. Confusing their principal clinical roles reverses the usual application of these radionuclides.

4. What happens to technetium-99m pertechnetate after it is taken up by the thyroid?

A. It is released without undergoing organification

B. It is organified and stored in thyroglobulin

C. It is retained until its radioactive half-life is complete

D. It is converted into thyroid hormone and secreted

Answer: (A)

Explanation

Technetium-99m pertechnetate enters the thyroid but is released without being organified. Iodine is the tracer that undergoes organification in the thyroid.

5. Which characteristic is associated with iodine-131?

A. It emits 159 keV photons and has an approximately 13.2-hour half-life

B. It emits 70 keV photons and has an approximately 2-hour half-life

C. It emits 511 keV photons and has an approximately 30-day half-life

D. It emits 364 keV photons and has an approximately 8-day half-life

Answer: (D)

Explanation

Iodine-131 emits 364 keV photons, has an approximately 8-day half-life, and is mainly therapeutic. The 159 keV photon energy and 13.2-hour half-life describe iodine-123.

6. A thyroid study measures tracer uptake at 20 minutes. Which value falls within the normal range for iodine-123 at that time?

A. A value between 0.3% and 3.9%

B. A value between 15% and 25%

C. A value between 30% and 40%

D. A value between 3% and 6%

Answer: (D)

Explanation

Normal iodine-123 uptake at 20 minutes is approximately 3–6%. The 0.3–3.9% range applies to technetium-99m, while 15–25% is the iodine-123 range at 4 hours.

7. Which scintigraphic pattern is usually associated with hyperthyroidism?

A. Hypofixation, because increased hormone production reduces tracer delivery

B. Uniformly normal fixation, because hormone excess does not alter tracer uptake

C. Absent fixation, because hyperfunctioning tissue cannot trap radionuclides

D. Hyperfixation, although iodine-related effects and thyroiditis can create exceptions

Answer: (D)

Explanation

Hyperthyroidism usually produces hyperfixation because functioning thyroid tissue traps more tracer. Thyroiditis and iodine-related hyperthyroidism are important exceptions that may produce reduced fixation.

8. How does a cold thyroid nodule differ from a hot nodule?

A. A cold nodule has diffuse glandular uptake, whereas a hot nodule represents inflammation around an inactive lesion

B. A cold nodule has increased fixation and autonomous activity, whereas a hot nodule has greater cancer concern

C. A cold nodule has normal fixation and minimal clinical significance, whereas a hot nodule lacks detectable tracer

D. A cold nodule has very low fixation and greater cancer concern, whereas a hot nodule usually reflects autonomous function

Answer: (D)

Explanation

Cold nodules show absent or very low tracer fixation and carry an estimated 5–20% cancer risk, whereas hot nodules fix tracer at least as well as surrounding tissue and usually function autonomously. Thus, low fixation creates the greater malignancy concern in this comparison.

9. When a cold thyroid nodule measures more than 10 mm, which evaluation strategy is appropriate?

A. Use radionuclide uptake measurement without structural imaging

B. Combine TSH testing, ultrasound, and fine-needle aspiration

C. Begin thyroid hormone treatment before obtaining tissue information

D. Observe the nodule with serial scans without biochemical testing

Answer: (B)

Explanation

Evaluation of a cold nodule larger than 10 mm combines TSH measurement, ultrasound, and fine-needle aspiration. A radionuclide scan alone cannot provide the biochemical, structural, and cytologic assessment needed for this nodule.

10. Which feature is directly assessed by thyroid scintigraphy?

A. Tracer avidity and the distribution of hot and cold areas

B. Renal filtration rate and urinary calcium excretion

C. Serum antibody concentration and pituitary hormone release

D. Bone mineral density and vertebral compression severity

Answer: (A)

Explanation

Thyroid scintigraphy evaluates tracer uptake, gland morphology and size, substernal extension, and the distribution of hot and cold regions. Serum antibodies and bone or renal measurements require other investigations.

11. Which clinical situation is an established indication for thyroid scintigraphy?

A. Confirming osteoporosis before initiating calcium supplementation

B. Assessing isolated kidney stones without thyroid abnormalities

C. Evaluating a thyroid nodule or hyperthyroidism such as Graves disease

D. Screening every patient with normal thyroid hormone concentrations

Answer: (C)

Explanation

Thyroid scintigraphy is indicated for thyroid nodules, hyperthyroidism, pretherapeutic assessment, and follow-up after thyroidectomy for differentiated cancer. Osteoporosis and isolated kidney stones are not primary indications for this examination.

12. Which collimator is required for I-131 thyroid imaging?

A. A high-energy general-purpose collimator

B. A low-energy high-resolution collimator

C. A medium-energy general-purpose collimator

D. A pinhole collimator designed for Tc-99m

Answer: (A)

Explanation

I-131 requires a high-energy general-purpose collimator because of its high-energy photons. Low-energy high-resolution collimators are preferred for higher-resolution Tc-99m and I-123 imaging, while medium-energy collimators permit faster acquisition with lower resolution.

13. What is the correct sequence for routine thyroid scintigraphy acquisition?

A. Inject about 20 mCi, measure the syringe, then image after 2 hours

B. Image the thyroid, inject the tracer, and measure the syringe after the scan

C. Measure the syringe, inject about 5 mCi, measure again, then image at 15 minutes

D. Measure the syringe once, inject the tracer, and perform SPECT-CT immediately

Answer: (C)

Explanation

The procedure includes pre-injection syringe measurement, injection of approximately 5 mCi or 185 MBq, post-injection measurement, and thyroid-centered imaging 15 minutes later. Tomographic imaging is added when clinically indicated rather than performed routinely at the start.

14. How does parathyroid hormone respond to changes in serum calcium?

A. PTH rises when calcium falls and is suppressed when calcium is sufficient or high

B. PTH remains stable because calcium concentration does not regulate its secretion

C. PTH rises in parallel with calcium regardless of the calcium concentration

D. PTH falls when calcium falls and rises when calcium is sufficient or high

Answer: (A)

Explanation

Low serum calcium stimulates PTH secretion, whereas sufficient or elevated calcium suppresses it to maintain calcium balance. The opposite response would disrupt rather than preserve calcium homeostasis.

15. Which protocol is appropriate for parathyroid scintigraphy in suspected hyperparathyroidism?

A. Immediate SPECT-CT before tracer injection and delayed imaging at 3 days

B. MIBI imaging after 24 hours, followed by ultrasound in every case

C. A single thyroid image at 15 minutes using only a low-energy tracer

D. MIBI imaging at 15–20 minutes and 1 hour 30 minutes, with SPECT-CT when needed

Answer: (D)

Explanation

Parathyroid scintigraphy uses MIBI images at approximately 15–20 minutes and 1 hour 30 minutes, with SPECT-CT when additional localization is required. The other schedules do not reflect the described parathyroid imaging protocol.

16. Which scintigraphic pattern most strongly supports Graves disease?

A. Focal hyperfixation with suppression of the remaining thyroid tissue

B. Diffuse hypofixation with high TSH and low T4 concentrations

C. Acute-phase hypofixation followed by spontaneous recovery within months

D. Diffuse homogeneous hyperfixation with a goiter and markedly suppressed TSH

Answer: (D)

Explanation

Graves disease produces diffuse homogeneous tracer hyperfixation, usually with diffuse goiter, very low TSH, and elevated T3 and T4. Focal uptake with suppression of the remaining gland is characteristic of a toxic autonomous nodule.

17. A patient over age 40 has focal thyroid hyperfixation with suppression of the rest of the gland. Which diagnosis best fits this pattern?

A. Acute viral thyroiditis

B. A toxic autonomous thyroid nodule

C. Primary Hashimoto hypothyroidism

D. Diffuse autoimmune Graves disease

Answer: (B)

Explanation

A toxic autonomous nodule produces focal hyperfixation while suppressing the remaining thyroid tissue and commonly occurs after age 40. Graves disease instead causes diffuse homogeneous fixation, whereas Hashimoto disease and viral thyroiditis typically cause hypofixation.

18. Which combination is characteristic of primary Hashimoto hypothyroidism?

A. High calcium, low phosphate, markedly elevated PTH, and a parathyroid adenoma

B. Low or normal TSH, low T4, focal hyperfixation, and suppressed surrounding tissue

C. High T3 and T4, very low TSH, diffuse hyperfixation, and a goiter

D. Positive anti-TPO and anti-thyroglobulin antibodies, diffuse hypofixation, high TSH, and low T4

Answer: (D)

Explanation

Primary Hashimoto hypothyroidism reflects progressive autoimmune destruction and is associated with positive thyroid antibodies, diffuse low uptake, elevated TSH, and low T4. The other combinations describe secondary hypothyroidism, Graves disease, or primary hyperparathyroidism.

19. How does secondary hypothyroidism from pituitary disease differ hormonally from primary hypothyroidism?

A. Secondary disease has low TSH with high T4, whereas primary disease has high TSH with high T4

B. Secondary disease has low or normal TSH with low T4, whereas primary disease usually has TSH above 10

C. Secondary disease has high TSH with low T4, whereas primary disease has normal TSH with high T4

D. Secondary disease has high TSH with high T4, whereas primary disease has low TSH with low T4

Answer: (B)

Explanation

Pituitary disease causes inadequate TSH stimulation, so TSH is low or inappropriately normal despite low T4. In primary hypothyroidism, thyroid failure typically causes compensatory TSH elevation, often above 10.

20. Which expression correctly calculates thyroid uptake as a percentage?

A. $\frac{\text{thyroid counts} + \text{background counts}}{\text{injected counts}} \times 100$

B. $\frac{\text{injected counts} - \text{thyroid counts}}{\text{background counts}} \times 100$

C. $\frac{\text{thyroid counts}}{\text{background counts} - \text{injected counts}} \times 100$

D. $\frac{\text{thyroid counts} - \text{background counts}}{\text{injected counts}} \times 100$

Answer: (D)

Explanation

Thyroid uptake is obtained by subtracting background counts from thyroid counts, dividing by injected counts, and multiplying by 100. Adding the background counts would exaggerate the apparent thyroid activity rather than correct for it.

21. During thyroid scintigraphy, where should the background region of interest be placed?

A. In equivalent cervical muscle adjacent to the thyroid

B. Across the thyroid gland at its region of greatest activity

C. Within the salivary glands near the angle of the jaw

D. Over the lower sternum beneath the thyroid bed

Answer: (A)

Explanation

The background ROI is placed in equivalent cervical muscle so that nearby non-thyroid activity can be estimated appropriately. Placing it over the gland would measure thyroid activity rather than background.

22. Which set of activities belongs to daily quality control for a Tc-99m thyroid imaging system?

A. Use a 20% window, administer 185 MBq, and interrupt breastfeeding

B. Replace the detector annually, measure patient TSH, and calculate thyroid uptake

C. Assess antibody levels, repeat imaging, and record the patient's radiation history

D. Verify the 140 ± 5 keV photopeak, test flood uniformity, and inspect the collimator

Answer: (D)

Explanation

Daily QC includes checking the Tc-99m photopeak, testing flood-field uniformity, and inspecting the collimator, along with other detector checks and a QC log. Patient laboratory results and administered dose are clinical or protocol information, not substitutes for these equipment checks.

23. A patient moves during thyroid imaging and has a metallic necklace near the neck; which interpretation is most appropriate for a resulting cold-appearing region?

A. It may be an artifact caused by movement or metal rather than abnormal tracer distribution

B. It indicates that the thyroid-to-background ratio has reached a normal value

C. It confirms a true cold thyroid area caused by reduced tracer accumulation

D. It demonstrates that salivary secretion has increased tracer concentration in the gland

Answer: (A)

Explanation

Movement can blur or double the image, and metallic jewelry can create cold artifacts that mimic abnormal tracer distribution. A true cold area reflects tracer distribution, so image quality and external objects must be considered before making that interpretation.

24. What is the appropriate approach to thyroid scintigraphy during pregnancy?

A. Delay all thyroid assessment until after delivery because blood tests are not informative

B. Use therapeutic I-131 at a reduced dose and monitor the fetus afterward

C. Perform the scan routinely with I-123 because laboratory tests cannot assess thyroid function

D. Avoid the scan except for a vital emergency and use TSH, T4, and antibody testing instead

Answer: (D)

Explanation

Pregnancy contraindicates I-123 and I-131 thyroid imaging except in a vital emergency, while biological assessment with TSH, T4, and antibodies remains available. Laboratory testing can evaluate thyroid function without exposing the pregnancy to the imaging radionuclides.

25. Which breastfeeding interruption is appropriate after administration of I-123 for thyroid imaging?

A. Interrupt breastfeeding until the patient's TSH concentration normalizes

B. Continue breastfeeding without a mandatory interruption

C. Interrupt breastfeeding for approximately 72–96 hours

D. Interrupt breastfeeding for approximately 2–4 weeks

Answer: (C)

Explanation

I-123 requires breastfeeding interruption for about 72–96 hours. Tc-99m generally requires no mandatory interruption, whereas therapeutic I-131 requires a substantially longer interruption of roughly 2–4 weeks.

26. Which pairing correctly matches an average administered dose with the corresponding thyroid scintigraphy agent?

A. Tc-99m: 200 μCi (7 MBq)

B. I-123: 5 mCi (185 MBq)

C. Therapeutic I-131: a fixed 200 μCi (7 MBq)

D. Tc-99m: 5 mCi (185 MBq)

Answer: (D)

Explanation

An average Tc-99m administered dose is about 5 mCi, equivalent to 185 MBq. I-123 is typically around 200 μCi, while therapeutic I-131 is dose-dependent and can deliver 15–100 Gy or more to the thyroid.