Thyroid

General

• Radiotracers

  • I-123

    • Only used in imaging

    • Decay via electron capture

    • T ½ = 13 hours

    • 159 keV

    • If breastfeeding, can resume 3 days after test

  • I-131

    • Used in imaging and treatment

    • Decay via beta emission

      • High radiation, so preferably not used in regular imaging because of higher radiation dose

    • T1/2 = 8 days

    • 364 keV

    • Should not be used in pregnant women

    • If breastfeeding, need to stop and discard any pumped milk

  • Tc-99m

    • Note you will see salivary glands in Tc study, will not see in I-123

    • Preferred over I-123 for imaging when

      • Receiving thyroid blocking agents (iodinated contrast)

      • Unable to take oral medication

      • Study must be completed in < 2 hours

    • If breastfeeding, can resume 12-24 hours after test

Iodine Uptake Test

• Purpose

  • Given before starting radioiodine therapy to ensure that the thyroid will uptake iodine and to determine how much it will uptake

  • Not used to diagnose hyper/hypothyroidism, but can differentiate between Graves, subacute thyroiditis and factitious hyperthyroidism

• Basically you give radioactive iodine and see how much is taken up by the thyroid to see how it normally handles iodine intake

  • High levels = more active thyroid gland

  • Low levels = less active thyroid gland

• Uptake

  • Given as a percentage of the administered activity in the thyroid gland at a specific time

• Normal Values

  • 6-8% for 4-6 hours

  • 10-30% for 24 hours

• Factors affecting uptake

  • High iodine diet = competition for the drug = artificially low tracer uptake

  • Low iodine diet = no competition for the drug = artificially increased tracer uptake

  • Drugs (anti-thyroid drugs)

  • Propranolol has NO EFFECT

  • Chronic kidney disease = do not clear iodine = higher iodine in body = competition for tracer = decreased tracer uptake

• Interpretation

  • Increased tracer uptake

    • Primary hyperthyroidism

      • Graves

      • Toxic nodular goiter (Plummer’s disease)

    • Secondary hyperthyroidism

  • Decreased tracer uptake

    • Primary hypothyroidism (gland does not respond to TSH)

    • Secondary hypothyroidism (pituitary does not secrete enough TSH)

Toxic Adenoma

• Treatment

  • First treatment should not exceed 33 mCi I-131

  • 30 mCi I-131 would be an appropriate first dose

Iodine Scan & MIBG

• Iodine scan

  • You should not see liver unless there is pathology present that would make it visible

    • I.e. If you have active thyroid tissue or mets that are metabolizing thyroglobulin then the by products are sent to the liver for further degradation and therefore the liver lights up

  • Iodine 131

  • Iodine 123 - preferred over I-131 because it does not have beta radiation

    • Beta radiation has no benefit or use in imaging but still acts as radiation so you would be giving a stronger amount of radiation which could be harmful to patient with no added benefit

• MIBG

  • You will see liver normally

  • If a kid, typical question asks about a neuroblastoma

• Need to be able to state dosing for Iodine ablation (high dose, low dose, cancer treatment dosing, etc)

Parathyroid Scan

• Agent = Tc-99m-sestamibi

  • Localizes in mitochondria of parathyroid cells

  • 20 mci IV

• Images at 15 min, 1 hour, 3 hours

• No real prep

• High sensitivity for primary hyperparathyroidism, lower for secondary

• Normal thyroid tissue will show uptake that gradually decreases over time

• Note: Parathyroid adenomas can occur really anywhere in the region of the neck so be sure to not only look posterior to the thyroid but also by esophagus, carotids, etc.

• Thyroid adenomas and parathyroid hyperplasia

  • Significantly increased uptake relative to the normal background thyroid but will also show decreased amount over time

• Parathyroid Adenoma

  • Significantly increased uptake relative to thyroid and will retain uptake on delayed images (hot quick and stays hot)

• If the delayed images are equivocal or unsure still —> inject with free Tc

  • If nodule gets hot —> thyroid adenoma

  • If node looks cold —> parathyroid adenoma

Parathyroid

• Typically looking for parathyroid adenoma

  • Most commonly located inferior or posterior to the thyroid

• Tc-99m Sestamibi - primary agent used most commonly

  • Fat soluble

  • Accumulates in cells with high mitochondrial activity

• Tc-99m tetrofosmin

  • Second line typically f

• F-18 Fluorocholine PET-CT

  • Increasingly used, typically when other study is inconclusive or it is a re-operative case

  • Higher sensitivity vs sestamibi

  • Uptake in cells with high phospholipid metabolism