Study sheet: Thyroid Scintigraphy and Nuclear Medicine

📋 Course Outline

1. Thyroid Function and Disorders
2. Radiopharmaceuticals and Tracer Uptake
3. Thyroid Scintigraphy Interpretation
4. Acquisition and Clinical Indications
5. Parathyroid Physiology and Imaging
6. Thyroid and Parathyroid Clinical Cases
7. Quality Control and Quantification
8. Artefacts, Safety, and Dosimetry

📖 1. Thyroid Function and Disorders

★ Must-know

• The thyroid secretes T3 (triiodothyronine) and T4 (thyroxine), regulates energy metabolism, contributes to growth and thermogenesis, and helps regulate body weight.

📌 Hyperthyroidism is characterized by decreased TSH with increased free T4 and free T3, whereas hypothyroidism is characterized by increased TSH with decreased free T4 and free T3.

Further detail

• Hypothyroidism can result from iodine deficiency, destruction of the gland by surgery, irradiation or thyroiditis, or disorders of hormone synthesis.

• Hyperthyroidism can result from:

  • Graves disease
  • acute thyroiditis
  • exogenous thyroid hormones
  • weight-loss medications

💡 Memory Hook

Hypothyroidism lowers T3/T4, whereas hyperthyroidism raises T3/T4.

📖 2. Radiopharmaceuticals and Tracer Uptake

📝 Essential Points

• Iodine-123 is primarily diagnostic, has a photon energy of 159 keV, and has a half-life of 13.2 hours.

• Technetium-99m pertechnetate is a common substitute that is taken up by the thyroid and then released without organification.

• Iodine-131 is mainly therapeutic, emits 364 keV photons, has an 8-day half-life, and is taken up and organified by the thyroid.

• Normal uptake at 20 minutes is 0.3–3.9% for Tc-99m, 3–6% for I-123, and I-123 uptake is 15–25% at 4 hours.

💡 Memory Hook

I-123 diagnoses, I-131 treats, and Tc-99m substitutes.

📖 3. Thyroid Scintigraphy Interpretation

★ Must-know

• Hypothyroidism usually causes hypofixation and hyperthyroidism usually causes hyperfixation, but iodine deficiency, iodine-induced hyperthyroidism and thyroiditis can produce exceptions.

• A cold nodule has absent or very low tracer fixation and carries a 5–20% cancer risk, whereas a hot nodule has equal or greater fixation than the surrounding thyroid and usually reflects autonomous function.

• For a cold nodule, evaluation combines TSH, ultrasound and fine-needle aspiration when the nodule exceeds 10 mm.

Further detail

• Isofixing nodules have an approximately 1–2% cancer risk, while hyperfixing nodules probably represent autonomous function.

💡 Memory Hook

Cold nodules require malignancy assessment, whereas hot nodules usually indicate autonomy.

📖 4. Acquisition and Clinical Indications

📝 Essential Points

• Thyroid scintigraphy evaluates:

  • tracer avidity
  • gland morphology
  • size
  • substernal extension
  • cold areas
  • hot areas

• Indications include: thyroid nodules, hyperthyroidism, pretherapeutic assessment, follow-up after thyroidectomy for differentiated cancer

📌 LEHR collimators provide high resolution for Tc-99m and I-123, MEGP collimators allow faster acquisition with lower resolution, and HEGP collimators are required for I-131.

• Thyroid scintigraphy consists of measuring the syringe before injection, injecting approximately 5 mCi (185 MBq), measuring the syringe afterward, acquiring a thyroid-centered image at 15 minutes post-injection, and adding tomographic imaging when indicated.

💡 Memory Hook

Prepare, inject, image, then correlate.

📖 5. Parathyroid Physiology and Imaging

🔑 Key Concepts & Definitions

• Parathyroid hormone : Maintains calcium balance by increasing when calcium falls and decreasing when calcium is sufficient or rises.

★ Must-know

• Parathyroid scintigraphy for suspected hyperparathyroidism uses MIBI imaging at 15–20 minutes and 1 hour 30 minutes, with SPECT-CT when required.

Further detail

• Hypocalcemia may cause muscle cramps, tetany, tingling and cardiac rhythm disorders, while long-term risks include osteoporosis or rickets.

• Hypercalcemia may cause:

  • fatigue
  • weakness
  • frequent urination
  • excessive thirst
  • nausea
  • constipation
  • confusion
  • kidney stones

💡 Memory Hook

Low calcium stimulates PTH release, whereas sufficient calcium suppresses PTH.

📖 6. Thyroid and Parathyroid Clinical Cases

📝 Essential Points

• Graves disease is caused by stimulating IgG antibodies against the TSH receptor and produces diffuse homogeneous hyperfixation, a diffuse goiter, very low TSH below 0.01, and elevated T3 and T4.

• A toxic autonomous nodule produces focal hyperfixation with suppression of the remaining thyroid, usually occurs after age 40, and may be treated with 10–20 mCi of I-131 or ablation.

• Primary Hashimoto hypothyroidism causes progressive autoimmune thyroid destruction, positive anti-TPO and anti-thyroglobulin antibodies above 200 IU, diffuse hypofixation below 0.5% with Tc-99m, and elevated TSH with low T4.

📌 Secondary hypothyroidism from pituitary disease presents with low or normal TSH and low T4, unlike primary hypothyroidism in which TSH is usually above 10.

• 🔄 Acute viral thyroiditis progresses through:
  1. thyrotoxicosis
  2. euthyroidism
  3. hypothyroidism

📌 Primary hyperparathyroidism is usually caused by an adenoma in 80% of cases and presents with increased calcium above 2.6, decreased phosphate and markedly increased PTH above 65 pg/mL.

📌 Secondary hyperparathyroidism is a compensatory response to chronic hypocalcemia, most often renal insufficiency, and causes normal or low calcium, markedly increased phosphate and increased PTH.

💡 Memory Hook

Graves is diffusely hyperfixing, whereas toxic nodules are focally hyperfixing.

📖 7. Quality Control and Quantification

★ Must-know

📐 Formula — Thyroid uptake is calculated as $(\text{thyroid counts} - \text{background counts}) / \text{injected counts} \times 100$.

• The thyroid ROI is drawn around the gland on the planar image, the background ROI is placed in equivalent cervical muscle, and a normal thyroid-to-background ratio is at least 2.

• Daily quality control includes: checking the Tc-99m photopeak at 140 ± 5 keV, performing flood-field uniformity testing, calibrating detector efficiency and sensitivity, inspecting the collimator, maintaining a mandatory QC log

Further detail

• Standard energy-window width is 20%, an acceptable energy shift is ±2%, acceptable daily short-term drift is below 0.5%, and long-term calibration is performed annually by the supplier.

💡 Memory Hook

Calibrate, acquire, measure, and document.

📖 8. Artefacts, Safety, and Dosimetry

★ Must-know

• Patient movement causes blurring or image doubling, neck flexion can create artifactual cold areas, and metallic jewelry can produce cold artifacts.

📌 Thyroid scintigraphy is absolutely contraindicated during pregnancy except for a vital emergency, and biological assessment with TSH, T4 and antibodies is the alternative.

• Tc-99m requires no mandatory breastfeeding interruption, I-123 requires interruption for 72–96 hours, and therapeutic I-131 requires interruption for 2–4 weeks.

• Average administered doses include 5 mCi (185 MBq) of Tc-99m, 200 μCi (7 MBq) of I-123, and dose-dependent therapeutic I-131 producing 15–100 Gy or more to the thyroid.

Further detail

• Salivary contamination from non-organified Tc-99m pertechnetate can falsely lower apparent thyroid uptake; prevention includes sucking lemon or an acidic candy for 10–15 minutes after injection.

💡 Memory Hook

Pregnancy contraindicates thyroid radiotracers, whereas breastfeeding requires tracer-specific interruption.

📊 Synthesis Tables

Thyroid Scintigraphy Patterns

Condition	TSH	Scintigraphic pattern
Graves disease	Very low	Diffuse homogeneous hyperfixation
Toxic autonomous nodule	Low	Focal hyperfixation with suppression elsewhere
Primary Hashimoto hypothyroidism	High	Diffuse hypofixation
Secondary hypothyroidism	Low or normal	Hypofixation