title: Increased need for thyroxine in women with hypothyroidism during estrogen therapy.
abstract: BACKGROUND
  Women with hypothyroidism that is being treated with thyroxine often need higher doses when they are pregnant. Whether this need can be attributed solely to estrogen-induced increases in serum thyroxine-binding globulin or whether other factors are involved is not known.
  
  
  METHODS
  In 11 postmenopausal women with normal thyroid function and 25 postmenopausal women with hypothyroidism treated with thyroxine, I assessed thyroid function before they started estrogen therapy and every 6 weeks for 48 weeks thereafter. The women with hypothyroidism included 18 women receiving thyroxine-replacement therapy and 7 women receiving thyrotropin-suppressive thyroxine therapy. On each occasion, serum thyroxine, free thyroxine, thyrotropin, and thyroxine-binding globulin were measured.
  
  
  RESULTS
  In the women with normal thyroid function, the serum free thyroxine and thyrotropin concentrations did not change, whereas at 12 weeks the mean (+/-SD) serum thyroxine concentration had increased from 8.0+/-0.9 microg per deciliter (103+/-12 nmol per liter) to 10.4+/-1.5 microg per deciliter (134+/-19 nmol per liter, P<0.001) and the serum thyroxine-binding globulin concentration had increased from 20.3+/-3.5 mg per liter to 31.3+/-3.2 mg per liter, P<0.001). The women with hypothyroidism had similar increases in serum thyroxine and thyroxine-binding globulin concentrations during estrogen therapy, but their serum free thyroxine concentration decreased from 1.7+/-0.4 ng per deciliter (22+/-5 pmol per liter) to 1.4+/-0.3 ng per deciliter (18+/-4 pmol per liter, P<0.001) and their serum thyrotropin concentration increased from 0.9+/-1.1 to 3.2+/-3.1 microU per milliliter (P<0.001). The serum thyrotropin concentrations increased to more than 7 microU per milliliter in 7 of the 18 women in the thyroxine-replacement group and to more than 1 microU per milliliter in 3 of the 7 women in the thyrotropin-suppression group.
  
  
  CONCLUSIONS
  In women with hypothyroidism treated with thyroxine, estrogen therapy may increase the need for thyroxine.
authors.affiliation: Division of Clinical and Molecular Endocrinology, Case Western Reserve University and the University Hospitals of Cleveland, OH 44106, USA.
authors.forename: B M
authors.initials: BM
authors.lastname: Arafah
mesh_terms: D000328:Adult; D004966:Estrogens, Conjugated (USP) / Q000494:pharmacology / Q000627:therapeutic use*; D005260:Female; D020249:Hormone Replacement Therapy; D006801:Humans; D007037:Hypothyroidism / Q000097:blood / Q000188:drug therapy*; D008525:Medroxyprogesterone / Q000627:therapeutic use; D008875:Middle Aged; D017698:Postmenopause / Q000097:blood; D012016:Reference Values; D013960:Thyroid Function Tests; D013961:Thyroid Gland / Q000187:drug effects / Q000502:physiology; D013964:Thyroid Neoplasms / Q000188:drug therapy; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000008:administration & dosage* / Q000097:blood / Q000627:therapeutic use; D013975:Thyroxine-Binding Proteins / Q000378:metabolism*
chemical_list: D004966:Estrogens, Conjugated (USP); D013975:Thyroxine-Binding Proteins; D013972:Thyrotropin; D008525:Medroxyprogesterone; D013974:Thyroxine