Radiopharmaceuticals for Thyroid

  1. A and P of the thyroid gland
    1. The thyroid gland is composed of a large number of follicles, each lined with epithelial cells and filled with colloid.
    2. The major constituent of the colloid is thyroglobulin, which is the base for production and storage of thyroid hormone.

    3. Ingested iodine is taken up from the blood by thyroid epithelial cells.
      1. Within these cells, iodide is rapidly oxidized to iodine by peroxidase enzyme and hydrogen peroxide.
      2. The iodine then reacts with tyrosine residues in thyroglobulin to form thyroid hormone, which is stored in the colloid.
      3. Trapping of iodide by epithelial cells is the rate-limiting step in hormone production. This active transport process is capable of concentrating iodide to 40 times the plasma concentration.
      4. A 10 μCi dose of I-131 sodium iodide contains only 8 x 10| -5ug of iodine (one eighty millionth of total body iodine).
      5. Review of the above diagram shows how
    4. Physiology - The follicular cells in the thyroid organify the iodine by peroxidases in the presence of hydrogen peroxide converting I- to I2
      1. Organic binding occurs on tyrosyl residue
      1. Tyrosyl is located on thyrogobulin (TBG) and forms monoiodotyrosine globulin (MIT)
      2. This occurs within the follicular spaces and forms the following:
        1. MIT + MIT = Diiodotyrosine. (DIT)
        1. MIT + DIT = Triiodothyronine (T3)
        2. DIT + DIT = Tetraiodothyronine - Thyroxine (T4)
      3. T3 and T4 are bound to TBG and circulate in the blood stream
        1.   All T4 is produced in the thyroid
        2. Only 20% of T3 is produced by the thyroid, while the remainder is produced peripherally via monodeiodination of T4
        3.   It is T3 that becomes the active ingredient that directly affects the metabolism of the body at the cellular level (not T4)
      4. Storage and released
        1. Thyroid hormones are stored with TBG in colloid form
        2. TSH stimulates the release of TBG and T4 from colloid form into the follicular cells by pinocytosis
        3. Hydrolysis of intracellular TBG releases T3 and T4 into circulation
        4. Note - it is the the free form of T3 that is active. In the bound form it is considered inactive (the active form is used in the bodies metabolism)
        5. Note - DIT, MIT and TBG are also released into circulation
      5. In a normal thyroid, T3 and T4 levels are affected by Thyrotropin-Releasing Hormone (TRH) and Thyroid Stimulating Hormone (TSH) production
        1. Excess levels of T3 and T4 cause reduced production of TRH and TSH
        2. Decreased levels T3 and T4 cause increased levels of TRH and TSH
        3. Since T3 and T4 cause an opposite reaction the term negative biofeedback loop is used
        4. Reduced levels of TRH and TSH reduces iodine trapping, resulting in reduced production of T3 and T4
        5. Increased levels of TRH and TSH increase iodine trapping, resulting increased production of T3 and T4
        6. Hence, increased production in one results in decreased production in the other, the effect being opposite or "negative"
        7. Refer to the diagram above to identify the process being discussed
  2. Sodium Iodide I-131
    1. Compounding – Available commercially in hard gelatin capsules and in aqueous solution for oral administration. Capsules for diagnostic studies are generally available in 15, 25, 50, and 100 μCi.
    2. The capsules contain sodium radioiodide, either mixed with polyethylene glycol and thiosulfate as a thin film on the inside surface of the capsule or mixed with granulated powder. Therapeutic capsules are also available.
    3. What is the major concern between liquid and capsule forms of radioiodine?
    4. Clinical use
      1. Radioactive Iodine Uptake (4 – 10 μCi)
      2. If uptake and scan are required, then the above dose is used for uptake and then followed with a 5 to 10 mCi dose of 99mTcO4-
      3. Consider the radiation dosimetry when comparing 131I to 123I
    5. Radioiodine Therapy (55 – 200 μCi/gram tissue).

  3. Sodium Iodide I-123
    1. Compounding – Capsules are available in 100 and 200 μCi sizes for diagnostic studies. Larger quanities can be obtained for whole imaging imaging.
    2. Clinical use – Thyroid scan (200 – 400 μCi)
  4. Biodistribution – Iodide is cleared from the plasma primarily by the thyroid gland but also by the salivary glands, gastric mucosal cells, mammary glands, and kidneys. It is widely distributed in the body after administration.
  5. Excretion – Renal clearance of iodide is by glomerular filtration. About 73% of filtered iodide is reabsorbed by the tubule.

  6. Tc-99m sodium pertechnetate
    1. Tc-99m sodium pertechnetate is trapped by the thyroid gland (i.e., transported into follicular cells of the thyroid) but is nor organified or synthesized into thyroid hormone.
    2. Anatomic imaging is performed with 2 – 10 mCi.

  7. Fludeoxyglucose F-18
    1. Used to localize recurrent disease (thryoid cancer) following surgery and I-131 therapy
    2. This approach occurs when the patients has a rising Tg and a negative I-123 or I-131 whole-body scan.   

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