What is the most likely cause of a 53-year-old female's symptoms of fatigue, weight gain, and hoarseness after a history of hyperthyroidism treated by radioactive iodine?

The symptoms of fatigue, weight gain, and hoarseness in this 53-year-old female patient can be attributed to hypothyroidism, particularly in the context of her prior treatment for hyperthyroidism with radioactive iodine. Radioactive iodine is a common treatment modality for hyperthyroidism, as it selectively ablates thyroid tissue to reduce hormone production. However, in some patients, this treatment can lead to an underactive thyroid (hypothyroidism) over time as the thyroid gland may become insufficient to meet the body's metabolic needs.

The specific symptoms presented align with typical manifestations of hypothyroidism. Fatigue is a frequent complaint due to decreased metabolic activity, weight gain can occur because of slowed metabolism, and hoarseness may result from thyroid enlargement or the impact of hypothyroidism on vocal cord function.

In this case, the history of hyperthyroidism followed by radioactive iodine treatment strongly suggests the development of hypothyroidism as a post-therapeutic complication. Therefore, this makes hypothyroidism the most likely cause of her symptoms.