• V F Koehler, M Reincke, and C Spitzweg.
• Medizinische Klinik und Poliklinik IV, Klinikum der Universität München, Ludwig-Maximilians-Universität München, München, Deutschland.
• Internist (Berl). 2018 Jul 1; 59 (7): 644-653.

AbstractThe 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. Signs and symptoms of hypothyroidism include fatigue, bradycardia, constipation and cold intolerance. In subclinical hypothyroidism, symptoms may be absent. Initiation of levothyroxine (T4) therapy not only depends on the level of TSH elevation, but also on other factors, such as patient age, presence of pregnancy or comorbidities. Treatment of patients with subclinical hypothyroidism is still a controversial topic. In general, thyroid hormone replacement therapy in non-pregnant adults ≤ 70 years is clearly indicated if the TSH concentration is >10 mU/l. Standard of care for treatment of hypothyroidism is T4 monotherapy. The biochemical treatment goal for T4 replacement in primary hypothyroidism is a TSH level within the reference range (0.4-4.0 mU/l). 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.

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