What Causes Low Thyroid Hormone Levels?

Hypothyroidism—the clinical state resulting from insufficient thyroid hormone production—can arise from a variety of underlying causes, broadly categorized as primary (originating in the thyroid gland itself), secondary (due to pituitary dysfunction), or tertiary (related to hypothalamic impairment). The most common cause worldwide is autoimmune thyroiditis, particularly Hashimoto’s thyroiditis, in which cytotoxic T lymphocytes and autoantibodies—such as anti-thyroid peroxidase (TPO) and anti-thyroglobulin antibodies—induce progressive lymphocytic infiltration and destruction of thyroid follicular cells.

Other significant etiologies include iodine deficiency—especially prevalent in regions without iodized salt programs—as iodine is an essential substrate for thyroxine (T4) and triiodothyronine (T3) synthesis. Iatrogenic causes are also frequent: post-thyroidectomy hypothyroidism, radioactive iodine-131 ablation for hyperthyroidism or thyroid cancer, and external beam radiation to the neck or upper mediastinum can all impair thyroid function permanently. Certain medications—including lithium, amiodarone, interleukin-2, and checkpoint inhibitors—may disrupt thyroid hormone synthesis, release, or immune regulation, leading to transient or permanent hypothyroidism.

Congenital hypothyroidism, often due to thyroid dysgenesis (e.g., ectopic gland, aplasia, or hypoplasia) or dyshormonogenesis (inborn errors of thyroid hormone biosynthesis), requires prompt newborn screening and lifelong levothyroxine replacement to prevent neurocognitive deficits. Central hypothyroidism—less common but clinically important—results from pituitary TSH deficiency (e.g., due to macroadenomas, Sheehan’s syndrome, or infiltrative diseases) or hypothalamic TRH deficiency, and is characterized by low or inappropriately normal serum TSH alongside low free T4 levels.

Subacute thyroiditis, silent thyroiditis, and postpartum thyroiditis may present with transient hypothyroidism following an initial thyrotoxic phase, reflecting destructive inflammation and subsequent hormone depletion. Rarely, resistance to thyroid hormone—a genetic disorder involving mutations in the thyroid hormone receptor beta gene—mimics biochemical hypothyroidism but is associated with elevated circulating thyroid hormones and non-suppressed TSH.

Accurate diagnosis hinges on measuring serum TSH as the initial screening test, followed by free T4 assessment when TSH is abnormal. Additional testing—including thyroid autoantibodies, thyroid ultrasound, or pituitary imaging—may be warranted depending on clinical context and initial results. Early recognition and appropriate management are critical to mitigating systemic consequences, including dyslipidemia, bradycardia, cognitive slowing, myxedema, and, in severe cases, myxedema coma.