Final answer:
The diagnosis based on high T4/T3, decreased TSH, and a single hot nodule on a RAIU scan is likely toxic multinodular goiter. Graves' disease, thyroiditis, and thyroid cancer present differently and are less consistent with the findings.
Step-by-step explanation:
The diagnosis based on the findings of high T4/T3, decreased TSH, and a single hot nodule on a Radioactive Iodine Uptake (RAIU) scan points towards toxic multinodular goiter. Toxic multinodular goiter is characterized by one or more autonomously functioning thyroid nodules that produce excess thyroid hormones, leading to hyperthyroidism. These nodules can be identified as 'hot' or hyperfunctioning areas on a RAIU scan, where they absorb more of the radioactive material than the surrounding thyroid tissue.
The diagnosis is not Graves' disease, as this condition typically presents with a diffusely enlarged thyroid gland and may include symptoms such as exophthalmos. Thyroiditis typically results in an initial phase of hyperthyroidism due to the release of stored thyroid hormones secondary to inflammation, but usually does not show a hot nodule on a RAIU scan. Thyroid cancer is less likely to present with hyperthyroidism and would not typically cause a hot nodule appearance on a RAIU scan.