Levothyroxine Therapy

Hypothyroidism develops when thyroid hormone action is inadequate. Primary hypothyroidism arises from the thyroid gland; secondary and tertiary forms arise from pituitary or hypothalamic disease. Thyroid loss or damage after surgery or radioactive iodine can also create a need for replacement. Low thyroid hormone slows metabolic processes and can affect cardiovascular, gastrointestinal, neurologic, skin, reproductive, and musculoskeletal function. In primary disease, reduced thyroid output removes negative feedback on the pituitary, so thyroid-stimulating hormone (TSH) rises. Levothyroxine is synthetic thyroxine (T4). It replaces or supplements endogenous T4, and tissues convert some T4 to triiodothyronine (T3). Thyroid hormones act through nuclear receptors to regulate gene transcription and protein synthesis. The therapeutic aim is a clinical and biochemical euthyroid state, not nonspecific stimulation or weight loss. Thyroid hormone must never be used to treat obesity or for weight loss: in euthyroid people, usual replacement doses do not reduce weight, and larger doses can cause serious or even life-threatening toxicity. T4 clears slowly, so symptoms and laboratory results do not change immediately after a dose adjustment. Improvement may lag for weeks or months, and...