Subclinical Hyperthyroidism due to Multinodular Goiter: This condition is characterized by the presence of multiple nodules in the thyroid gland, some of which may be functioning autonomously, leading to suppressed TSH levels despite normal T3 and T4 levels. The thyroid scan showing areas of increased and suppressed uptake supports this diagnosis, as it indicates the presence of both hyperfunctioning and hypofunctioning thyroid tissue.

Toxic Multinodular Goiter: Similar to subclinical hyperthyroidism due to multinodular goiter, but with the potential for more pronounced symptoms of hyperthyroidism. The absence of elevated T3 and T4 levels and negative TSH receptor antibodies makes this less likely than subclinical hyperthyroidism but still a consideration.
Thyroiditis (e.g., Silent Thyroiditis): Although thyroiditis typically presents with a phase of hyperthyroidism followed by hypothyroidism, some forms like silent thyroiditis can have a prolonged hyperthyroid phase with suppressed TSH and normal T3 and T4 levels. The thyroid scan can show patchy uptake, but the clinical context and negative TSH receptor antibodies are key to differentiating this from other causes.

Thyroid Cancer with Functional Metastases: Although rare, the presence of multiple areas of increased uptake on the thyroid scan could theoretically represent metastatic thyroid cancer that is functioning and suppressing TSH. This diagnosis is critical to not miss due to its significant implications for treatment and prognosis.
Pituitary or Hypothalamic Disease: Central causes of hyperthyroidism, such as a TSH-secreting pituitary adenoma, are rare but critical to diagnose accurately. The normal T3 and T4 levels with suppressed TSH might suggest a central cause, especially if other pituitary hormone abnormalities are present.

Struma Ovarii: A rare ovarian tumor that contains thyroid tissue, which can produce thyroid hormones and suppress TSH. This diagnosis would be considered if there were ovarian masses and the thyroid scan showed little to no uptake in the thyroid gland.
Familial Dysalbuminemic Hyperthyroxinemia: A rare condition characterized by abnormal binding of T4 to albumin, leading to elevated total T4 but normal free T4 and T3 levels. However, this condition typically does not suppress TSH, making it less likely in this scenario.
Resistance to Thyroid Hormone (RTH): A rare genetic disorder where there is reduced sensitivity to thyroid hormones, leading to elevated T3 and T4 levels with a nonsuppressed TSH. However, some forms of RTH can present with suppressed TSH, especially in the context of other hormone abnormalities.