Decreased levels of TSH may be due to primary hyperthyroidism, secondary hypothyroidism (pituitary disease), tertiary hypothyroidism (hypothalamic disease), subclinical hyperthyroidism (due to toxic mutlinodular goiter or treated Graves disease), euthyroid sick syndrome or over-replacement of thyroid hormone in treated hypothyroid patients. The gene discussed is TG; the disease is hypothyroidism.