What are the causes of hyperthyroidism?
Hyperthyroidism causing symptoms (thyrotoxicosis) can be primary, secondary, or tertiary.
In primary hyperthyroidism, the thyroid produces T3 and T4 independently of stimulation by TSH. The most common cause of hyperthyroidism is Graves’ disease.
This typically occurs in women 20–40 years of age. In this disease, autoantibodies to thyroid TSH receptors result in stimulation of hormone production. Thyroiditis also causes release of thyroid hormones and may be either subacute (probably viral infection) or acute suppurative (bacterial infection). Toxic solitary nodules and toxic multinodular goiters may cause primary hyperthyroidism. A rare cause of primary hyperthyroidism is a molar preg-nancy secreting chorionic gonadotropin, which has TSH-like activity. The laboratory abnormalities in primary hyperthy-roidism are elevated levels of T3 and T4 and low levels of TSH.
In secondary hyperthyroidism, the pituitary produces excessive TSH, independent of stimulation by TRH. Pituitary adenomas are the most common cause of secondary hyper-thyroidism. The laboratory abnormalities in secondary hyperthyroidism are elevated levels of TSH, T3 and T4, but reduced levels of TRH.
Tertiary hyperthyroidism results from excessive secre-tion of TRH. The laboratory abnormalities in tertiary hyperthyroidism are elevated levels of TRH, TSH, and T3 and T4.