Radioactive Iodine Uptake in Thyroid diseases

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It's wise to remember that not all cases of hypothyroidism are associated with decreased Iodine uptake and not all cases of hyperthyroidism are associated with increased Iodine uptake.

Examples of hyperthyroidism with increased uptake:

• Typically Grave's disease as the autoantibodies stimulate TSH receptor sensitivity enhancing the uptake.
• Secondary and tertiary hyperthyoroidism where the elevated TSH levels also stimulate the uptake.
• Toxic adenoma in which the overactive adenoma need to take Iodine to support the production of T4.

Example of hyperthyroidism with decreased uptake:

• Factitious hyperthyroidism in which the exogenous T4 causes feedback inhibition on the TSH secretion.
• Release of thyroid hormone by destructive process e.g. temporary hyperthyroidism seen in subacute thyroiditis.

Example of hypothyroidism with increased uptake:

• Thyroid hormone receptor insensitivity in which the T4 has no peripheral effects (hence the hypothyroidism) and no feedback effects (hence the elevated TSH and increased uptake).

Example of hypothyroidism with decreased uptake:

• Hashimoto, De Quervain, and Reidle thyroditis are all associated with decreased Iodine uptake.

Thyroid carcinoma may variable be hyperthyroid/hypothyroid and Cold/Hot on radioactive Iodine uptake tests.