Hypothyroidism

Hypothyroidism is a deficiency of thyroid hormone (T4 and T3), the hormones that set the body's metabolic rate. With too little hormone, metabolic processes slow throughout the body, producing the classic picture of fatigue, cold intolerance, weight gain despite unchanged appetite, constipation, dry skin, hair thinning, bradycardia, and delayed relaxation of deep tendon reflexes.

Most cases are primary hypothyroidism — failure of the thyroid gland itself. Hashimoto thyroiditis, an autoimmune destruction of the gland, is the leading cause in iodine-sufficient regions, while iodine deficiency remains the most common cause worldwide. Other causes include thyroid surgery, radioactive iodine therapy, and certain medications. Central (secondary) hypothyroidism, from pituitary or hypothalamic disease, is far less common.

Laboratory patterns make the diagnosis. In primary hypothyroidism, TSH is elevated as the pituitary tries to stimulate a failing gland, while free T4 is low; in central disease, TSH is low or inappropriately normal alongside low free T4. Severe, longstanding hypothyroidism can cause myxedema — nonpitting edema from accumulated glycosaminoglycans — and, rarely, life-threatening myxedema coma. Treatment is replacement with synthetic T4 (levothyroxine), dosed to normalize TSH.

Hypothyroidism spans several health-professions exams: the USMLE Step 1 tests causes and TSH/T4 lab patterns, the PTCE covers levothyroxine and other thyroid medications, and the NPTE-PTA includes hypothyroidism among endocrine conditions relevant to rehabilitation.