CHANGES IN THE PHYSIOLOGY OF THE THYROID GLAND IN PREGNANCY AND THE NEED FOR TRIMESTER-SPECIFIC REFERENCE VALUES IN THYROID STATUS: A REVIEW

Abstract

Pregnancy is a unique state of the female organism in which occur physiological changes that are
necessary to maintain the pregnancy. To be able to respond to the increased metabolic needs of pregnancy and to
maintain an adequate level of thyroid hormones in the blood, the thyroid gland responds with changes in its function
and anatomy as well. Changes in thyroid function are mainly associated with increased levels of thyroxine-binding
globulins in the blood. Their level increases as a result of estrogen stimulation in pregnancy and they are synthesized
in the liver. Changes in the physiology of the thyroid gland during pregnancy also occur as a result of stimulation of
the TSH receptors of the gland by the human chorionic gonadotropin hormone (hCG) which is secreted by the
placental trophoblast.
Due to the difference in the physiology of the thyroid gland in and outside of pregnancy, a special interpretation of
the results of functional thyroid tests is required. Changes in hormone values are transitory, especially expressed in
the first trimester of pregnancy. A healthy pregnant patient without thyroid pathology or disease compensates for
those changes and a sufficient amount of thyroid hormones is produced. The presence of thyroid hormones in an
optimal concentration in the blood maintains a successful pregnancy and secures normal neurological development
of the fetus. Diseases of the thyroid gland lead to a decrease its compensatory ability during pregnancy and the
contributes to the inability to respond to the increased metabolic needs that follow.
The motive for this paper are the pregnant patients with undiagnosed or unrecognized thyroid gland diseases before
pregnancy, where we encounter conditions when the thyroid gland cannot respond to the physiological changes that
occur. One of the negative outcomes of this condition can be spontaneous pregnancy loss in the first trimester.
Therefore, a special interpretation of the thyroid status in pregnancy is needed and the introduction of constant
reference values that will apply only to pregnancy.
World recommendations designate reference values for TSH and thyroid hormones respectively for each trimester of
pregnancy, the so-called trimester-specific reference values. In case of absence of trimester-specific reference
values, there are recommendations regarding the special interpretation of thyroid function tests and reference values;
as well as recommendations for using adjusted values according to demographic characteristics that will apply to all
trimesters. This review of literature recommendations would help in the management of patients in order to achieve
an optimal serum level of thyroid-stimulating hormone and achieve a successful pregnancy, thus imposing the need
to introduce recommendations in our laboratories for thyroid analysis values specific to pregnancy, which will differ
from those for the non-pregnant population