In patients with hypothyroidism, which of the following tests would be used to monitor thyroid function?

In patients with hypothyroidism, monitoring thyroid function is primarily achieved through the measurement of thyroid-stimulating hormone (TSH) levels. TSH is produced by the pituitary gland and regulates the production of thyroid hormones (T3 and T4) from the thyroid gland. In individuals with hypothyroidism, TSH levels will typically be elevated due to the reduced levels of circulating thyroid hormones.

The rationale for using TSH as the primary test for monitoring is that it serves as an indicator of the body's feedback mechanism in response to thyroid hormone levels. If TSH levels decrease, it suggests that thyroid hormone levels are improving, and conversely, if TSH levels remain high, it indicates that hypothyroidism may still be present or inadequately treated.

Other tests such as T3 and T4 can provide additional information regarding thyroid hormone levels, but they are not as reliable as TSH for monitoring treatment efficacy in hypothyroidism. Specifically, T3 levels may not always accurately reflect the thyroid function status, especially in cases where T4 is primarily affected. Therefore, testing TSH is the standard approach for evaluating thyroid function in patients with hypothyroidism.