Primary hypothyroidism is commonly treated with prescription supplementation of synthetic thyroxine.
Because not all immunoassays for free T4 are equivalent at measuring synthetic forms of synthetic thyroid hormone, adequacy of thyroid hormone supplementation is monitored using TSH. Recall that as circulating T4 levels rise, the design of the negative feedback loop should be the pituitary responding with a decreased production of TSH levels. Thus, normalized TSH levels in conjunction with the improvement of hypothyroid-related symptoms serve as a guide to physicians that the dosing is adequate. If TSH levels fall below the therapeutic range, the amount of thyroid hormone treatment may be reduced, or if the TSH levels remain elevated, thyroid hormone supplementation may be increased. Monitoring frequency is often patient-specific, but is typically performed on a monthly to semi-annual basis, depending on symptoms.
Note that such hormone supplementation to treat hypothyroidism with ongoing monitoring is likely a life-long course, unless an underlying cause of the lack of hormone production could be identified and corrected.