A patient presents with low thyroid hormone levels and low TSH. What is the most likely diagnosis?

The scenario presented involves low thyroid hormone levels alongside low TSH levels, suggesting a disruption in the normal feedback loop between the thyroid gland and the pituitary gland. Under normal physiological conditions, when thyroid hormone levels are low, the pituitary gland should secrete more TSH to stimulate the thyroid to produce more hormones. However, in this case, both the thyroid hormones and TSH levels are low.

Hypopituitarism is characterized by inadequate secretion of pituitary hormones, which can lead to a deficiency in TSH production. When TSH is low, the thyroid gland does not receive the stimulation it requires to produce sufficient thyroid hormones, resulting in a clinical picture of low thyroid function and low TSH levels. This aligns perfectly with the patient's presentation.

In contrast, primary hypothyroidism typically presents with high TSH levels due to the feedback mechanism failing only at the thyroid level, while excessive dosing of levothyroxine would usually suppress TSH without concurrently lowering thyroid hormones significantly. Subacute thyroiditis also typically presents with an inflammatory response and may have variable TSH levels. Therefore, the clinical presentation strongly supports hypopituitarism as the most likely diagnosis.