Radioactive iodine uptake testing is a useful diagnostic tool for assessing thyroid pathologies.
The atom is the smallest unit of an element. An atom consists of electrons, protons and neutrons. Electrons revolve around the nucleus (made of protons and neutrons) in fixed orbits. The number of electrons equals number of protons in a neutral atom. Electrons can be entirely removed from an atom by absorption of energies that exceed the binding energies of the electrons, causing the atom to be ionized.
These excited atoms can release energy in the form of electromagnetic radiation. Electromagnetic force manifests itself as waves or particles called photons. A nuclide that releases radiation is called a radionuclide.
In the nuclide (nucleus), the number of neutrons (N) plus the number of protons (Z) equal its mass number (A). In the case of iodine-131 (131 I), the atomic number Z (number of protons) is 53 and number of neutrons is 78. So the total mass number is 131. An unstable nuclide tries to attain stability through radioactive decay releasing alpha, beta and gamma rays. In medicine, this radiation is used for diagnosis and treatment.
A normal thyroid gland traps about 10-35 % of ingested radioiodine through the sodium-iodide symporter (NIS).
NIS is also present in the salivary glands and breast, but only the thyroidal NIS is regulated by the thyroid-stimulating hormone (TSH). Thyroidal NIS cannot distinguish between radioactive and non-radioactive iodine (iodine-127). More than 20 radionuclide of iodine are recognized, but only iodine-123 (123 I) and131 I are commonly used for clinical purposes. On certain occasions iodine-124 and iodine-125 are used.
Because123 I has a comparatively short half-life and emits only (gamma) photons, it is ideal for diagnostic testing.131 I has a half-life of 8 days and emits beta rays as well as gamma rays. This makes it useful for diagnostic and therapeutic purposes. But the use of131 I for routine diagnosis is discouraged because the radiation dose is about 100 times stronger than that of123 I.
Other noniodide radionuclides are used for the evaluation of patients with thyroid disorders include the following:
Technetium pertechnetate (99mTcO4): This is more widely used than123 I because of availability and cost.
Unlike123 I, which is both concentrated and organified within the thyroid, technetium pertechnetate is only concentrated in the thyroid.
111In-pentreotide: This is a somatostatin analogue used in imaging medullary thyroid cancer and other neuroendocrine tumors.
See the images below.
Five different scintigrams taken from thyroids with different syndromes: (A) Normal thyroid; (B) Graves disease, diffuse increased uptake in both thyroid lobes; (C) Plummer disease (toxic multinodular goiter); (D) Toxic adenoma; (E) Thyroiditis (marker 99Tc).