Der Internist
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The diagnosis of hypothyroidism is primarily based on clinical signs and symptoms as well as measurement of thyroid-stimulating hormone (TSH) concentration. Subclinical hypothyroidism is characterized by elevated TSH with normal serum free thyroxine (fT4) and triiodothyronine (fT3) levels, while in manifest hypothyroidism serum fT4 and fT3 levels are reduced. Common causes of primary hypothyroidism are autoimmune thyroiditis as well as therapeutic interventions, such as thyroid surgery or radioiodine therapy. ⋯ In contrast, in secondary hypothyroidism, serum fT4 levels are the basis for adjusting thyroid hormone dosage. Inadequate replacement of T4 resulting in subclinical or even manifest hyperthyroidism should urgently be avoided. T4/liothyronine (T3) combination therapy is still a matter of debate and not recommended as standard therapy, but may be considered in patients with persistence of symptoms, despite optimal T4 treatment, based on expert opinion.