Which hormones does the thyroid secrete?
The thyroid secretes T3 and T4.
What are the main functions of thyroid hormones?
They regulate energy metabolism, growth, thermogenesis, and body weight.
How is hyperthyroidism characterized in terms of TSH and thyroid hormones?
By decreased TSH and increased free T4 and free T3.
How is hypothyroidism characterized in terms of TSH and thyroid hormones?
By increased TSH and decreased free T4 and free T3.
Name a cause of hypothyroidism related to iodine.
Iodine deficiency can cause hypothyroidism.
What gland-related causes can lead to hypothyroidism?
Destruction of the gland by surgery, irradiation, or thyroiditis can cause hypothyroidism.
What are some causes of hyperthyroidism?
Graves disease, acute thyroiditis, or exogenous thyroid hormones and weight-loss medications cause hyperthyroidism.
What is the primary use of Iodine-123 in nuclear medicine?
It is primarily diagnostic.
What is the photon energy of Iodine-123?
159 keV.
What is the half-life of Iodine-123?
13.2 hours.
Which radiopharmaceutical is taken up by the thyroid and then released without organification?
Technetium-99m pertechnetate.
What is the main use of Iodine-131?
It is mainly therapeutic.
What is the photon energy emitted by Iodine-131?
364 keV photons.
What is the half-life of Iodine-131?
8 days.
What is the normal uptake range of Iodine-123 at 4 hours?
15–25%.
What fixation pattern does hypothyroidism usually cause on thyroid scintigraphy?
Hypothyroidism usually causes hypofixation.
Which conditions can cause exceptions to typical thyroid scintigraphy fixation patterns?
Iodine deficiency, iodine-induced hyperthyroidism, and thyroiditis can cause exceptions.
What defines a cold nodule on thyroid scintigraphy?
A cold nodule has absent or very low tracer fixation.
What is the cancer risk associated with a cold nodule?
Cold nodules carry a 5–20% cancer risk.
What fixation pattern does a hot nodule show compared to surrounding thyroid tissue?
A hot nodule has equal or greater fixation than the surrounding thyroid.
What is the cancer risk of isofixing nodules?
Isofixing nodules have an approximately 1–2% cancer risk.
What do hyperfixing nodules probably represent?
Hyperfixing nodules probably represent autonomous function.
What evaluations are combined for a cold nodule larger than 10 mm?
TSH, ultrasound, and fine-needle aspiration are combined for evaluation.
What does thyroid scintigraphy evaluate in the gland?
Tracer avidity, morphology, size, substernal extension, and cold/hot area distribution.
Name one clinical indication for thyroid scintigraphy.
Thyroid nodules.
Which collimator type provides high resolution for Tc-99m and I-123?
LEHR collimators.
Which collimator type is required for I-131 imaging?
HEGP collimators.
What is the approximate injected activity for thyroid scintigraphy?
Approximately 5 mCi (185 MBq).
How does parathyroid hormone respond to low calcium levels?
It increases when calcium falls.
What symptoms can hypocalcemia cause?
Muscle cramps, tetany, tingling, and cardiac rhythm disorders.
What are long-term risks of hypocalcemia?
Osteoporosis or rickets.
Name common symptoms of hypercalcemia.
Fatigue, weakness, frequent urination, excessive thirst, nausea, constipation, confusion, and kidney stones.
What imaging technique is used in parathyroid scintigraphy for hyperparathyroidism?
MIBI imaging at 15–20 minutes and 1 hour 30 minutes.
When is SPECT-CT used in parathyroid scintigraphy?
When required for suspected hyperparathyroidism.
What causes Graves disease at the antibody level?
Stimulating IgG antibodies against the TSH receptor cause Graves disease.
How does a toxic autonomous thyroid nodule appear on imaging?
It produces focal hyperfixation with suppression of the remaining thyroid.
What antibodies are positive in primary Hashimoto hypothyroidism?
Anti-TPO and anti-thyroglobulin antibodies above 200 IU are positive.
How does secondary hypothyroidism differ in TSH levels from primary hypothyroidism?
Secondary hypothyroidism has low or normal TSH, unlike primary which has TSH above 10.
What is the typical course of acute viral thyroiditis?
It evolves through thyrotoxicosis, euthyroidism, and hypothyroidism with spontaneous resolution in 3–6 months.
What is the main cause of primary hyperparathyroidism?
An adenoma causes 80% of primary hyperparathyroidism cases.
What calcium and PTH changes occur in primary hyperparathyroidism?
Calcium is increased above 2.6 and PTH is markedly increased above 65 pg/mL.
What causes secondary hyperparathyroidism and its typical calcium level?
It is caused by chronic hypocalcemia, often renal insufficiency, with normal or low calcium.
How is thyroid uptake calculated?
Thyroid uptake equals (thyroid counts minus background counts) divided by injected counts times 100.
Where is the thyroid ROI drawn on a planar image?
Around the thyroid gland.
Where is the background ROI placed for thyroid imaging?
In equivalent cervical muscle.
What is the minimum normal thyroid-to-background ratio?
At least 2.
What daily quality control checks are included for Tc-99m imaging?
Checking the Tc-99m photopeak at 140 ± 5 keV, flood-field uniformity, detector calibration, collimator inspection, and maintaining a QC log.
What is the standard energy-window width for quality control?
20%.
What is the acceptable energy shift during daily quality control?
±2%.
How often is long-term calibration performed and by whom?
Annually by the supplier.
What image artifact does patient movement cause in thyroid scans?
Blurring or image doubling.
How can salivary contamination affect thyroid uptake measurements?
It falsely lowers apparent thyroid uptake.
What is the absolute contraindication for thyroid scintigraphy?
Pregnancy except for a vital emergency.
What is the alternative to thyroid scintigraphy during pregnancy?
Biological assessment with TSH, T4, and antibodies.
Which radionuclide requires no mandatory breastfeeding interruption?
Tc-99m.
How long must breastfeeding be interrupted after I-123 administration?
For 72–96 hours.
What is the typical administered dose of Tc-99m for thyroid imaging?
5 mCi (185 MBq).
What dose range does therapeutic I-131 deliver to the thyroid?
15–100 Gy or more, dose-dependent.
Test your knowledge with 26 questions on Thyroid Scintigraphy and Nuclear Medicine.
1. Which laboratory pattern most strongly indicates primary hyperthyroidism?
2. Which combination correctly describes major functions of the thyroid gland?
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