Euthyroidism associated with reduced TSH

Situations where euthyroid states may be associated with reduced TSH include:

• physiological causes
  • pregnancy - during pregnancy, near the end of the first trimester the concentration of human chorionic gonadotrophin (HCG) (with its thyroid stimulating activity) reaches a peak. Thus there is a suppression of TSH for a few weeks. This effect is exacerbated if there is hyperemesis gravidarum where there are higher concentrations of HCG and hence consequently raised concentrations of triiodothyronine and thyroxine
  • elderly patients - there is decreased thyroxine clearance and therefore suppression of production of TSH
• non-thyroid illness
  • in patients with severe non-thyroidal illness the commonest form includes low TSH levels with reduced triidodothyronine levels (occasionally with also reduced thyroxine levels). This effect may be due to factors interfering with peripheral thyroid hormone metabolims and conversion of thyroxine to triiodothyronine (e.g. cortisol) in conjunction with central TSH suppression (e.g. due to somatostatin)