REVIEW ARTICLE 475 Indian Journal of Clinical Practice, Vol. 20, No. 6, November 2009 Thyroid Disorders in Pregnancy Pralhad Kushtagi*, Prashanth Adiga** T hyroid disorders are the second most common endocrine disorders affecting women of reproductive age, and obstetricians often care for patients who have been previously diagnosed with alterations in the thyroid gland function. Disorders of thyroid hormone production can affect fertility, fetal growth and development. e physiological changes during pregnancy such as increase in cardiac output, oxygen consumption and heat production may mimic mild thyrotoxicosis, may exacerbate or improve underlying thyroid disorder. Thyroid Physiology During Pregnancy It is imperative for physicians to be aware of changes that occur in thyroid physiology during pregnancy. In addition to changes in maternal thyroid elaboration, one also needs to keep in mind the fetal milieu (Table 1). Maternal Serum thyroxine binding globulin (TBG) levels rise in pregnancy due to estrogen mediated increases in hepatic synthesis and greater sialation, with consequent decreased hepatic clearance of TBG. 1 High levels of deiodinases types II and III are found in the placental tissue, such that in pregnancy there is, proportionally, greater production of biologically   inactive reverse tri-iodothyronine (rT3) than in non-pregnant state. 2 Normal pregnancy is associated with increased glomerular filtration rate, such that the renal clearance of iodides increases. is may be sufficient to decompensate women with marginal iodine deficiency, leading to goiter. 3,4 Serum free thyroxine (fT3) and thyroxine stimulating hormone (TSH) are considered as the most reliable indicators of thyroid function during pregnancy. 5 Fetal e developing pituitary-thyroid axis is sufficiently mature to function, towards the end of the first trimester, with fetal derived T 4 detectable in blood from around 10 week’s gestation. 2 Fetal serum levels of TSH, TBG, fT 4 and fT 3 increase throughout gestation, reaching mean adult levels at approximately 36 weeks of gestation. Placental transfer of thyroid occurs, which is explained by the fact that significant concentrations of T 4 and T 3 are detectable in blood of neonates unable to synthesize thyroid hormones due to congenital organification defects. 4     ABSTRACT yroid disorders are the second most common endocrine disorders affecting women of reproductive age. Disorders of thyroid hormone product affect fertility, pregnancy outcome, fetal growth and development. Key words: Serum thyroxine binding globulin, free thyroxine, thyroid stimulating hormone, thyroid storm *Professor **Associate Professor Dept. of Obstetrics and Gynecology, Kasturba Medical College, Manipal Address for correspondence Dr Pralhad Kushtagi 1, KMC Quarters Manipal - 576 104 E-mail: pralhadkushtagi@hotmail.com Table 1. Changes in Thyroid Physiology in Pregnancy Serum TBG increases  Total T 4 and T 3 rise  Increased clearance of iodide  Free T 3 and TSH are the best indicators of thyroid function  REVIEW ARTICLE