Hypothyroidism

Hypothyroidism is inadequate thyroid hormone effect. The diagnostic question is not simply whether a patient feels tired or has gained weight; it is whether free T4 is low and whether the pituitary has mounted an appropriate TSH response. In primary hypothyroidism, the thyroid gland fails, most often because of chronic autoimmune thyroiditis in an iodine-sufficient setting. Falling thyroid hormone removes negative feedback, so TSH rises. In central hypothyroidism, pituitary or hypothalamic disease prevents that appropriate rise; free T4 is low but TSH may be low or appear normal. That distinction changes both the diagnostic test strategy and how treatment is monitored. A patient with fatigue, constipation, dry skin, and cold intolerance may warrant testing, but symptoms alone do not establish the diagnosis. Conversely, a normal TSH does not exclude central disease, and a mildly abnormal TSH should be interpreted with the free T4, laboratory reference interval, clinical context, pregnancy status, medications, and recent illness.