Thyroid and parathyroid

  • Thyroxine-binding globulin (TBG) concentrations rise due to increased oestrogen levels.
  • T4 and T3 increase over the first half of pregnancy but there is a normal to slightly decreased amount of free hormone due to increased TBG-binding.
  • TSH production is stimulated, although in healthy individuals this is not usually significant. A large rise in TSH is likely to indicate iodine deficiency or subclinical hypothyroidism.
  • Serum calcium levels decrease in pregnancy, which stimulates an increase in parathyroid hormone (PTH).
  • Colecalciferol (vitamin D3) is converted to its active metabolite, 1,25-dihydroxycolecalciferol, by placental 1-hydroxylase.