Early pregnancy reference intervals of thyroid hormone concentrations in a thyroid antibody-negative pregnant population.
BACKGROUND
Thyroid dysfunction and antibodies are increasingly recognized as risk factors during pregnancy. Thyroid function changes during pregnancy and there is a need for gestational age-specific reference intervals for thyroid hormones. The aim of this study was to calculate gestational age-specific thyrotropin (TSH), free thyroxine (fT4), and free triiodothyronine (fT3) reference intervals in an iodine-sufficient thyroid antibody-negative population.


METHODS
The study population consisted of a large, prospective population-based cohort, the Northern Finland Birth Cohort 1986 (singleton births, n = 9362), with extensive data throughout gestation. The subjects underwent serum sampling in early pregnancy. Samples were assayed for TSH, fT4, fT3, thyroid-peroxidase, and thyroglobulin antibodies (n = 5805). All mothers with thyroid antibodies or previous thyroid diseases were excluded when calculating gestational age-specific percentile categories for TSH, fT4, and fT3. Also, associations between body mass index (BMI) and thyroid hormones were established.


RESULTS
The upper reference limit for TSH was 2.5 multiples of median (2.7-3.5 mU/L, depending on gestational week). The lower reference limit was as low as 0.07 mU/L. Reference intervals for fT4 rose during early pregnancy and decreased thereafter, ranging between 11-22 pmol/L. Reference intervals for fT3 were uniform throughout gestation, ranging between 3.4 and 7.0 pmol/L. BMI was associated positively with early pregnancy TSH and fT3 concentrations and negatively with fT4 concentrations.


CONCLUSIONS
These gestational age-specific reference intervals for thyroid hormones provide a framework for clinical decision making. Overweight and obesity are increasing problems among fertile women and they are associated with possibility of thyroid dysfunction during pregnancy.
Department of Obstetrics and Gynecology, University of Oulu, Oulu, Finland.
Tuija
T
Männistö
Heljä-Marja
HM
Surcel
Aimo
A
Ruokonen
Marja
M
Vääräsmäki
Anneli
A
Pouta
Aini
A
Bloigu
Marjo-Riitta
MR
Järvelin
Anna-Liisa
AL
Hartikainen
Eila
E
Suvanto
D001323:Autoantibodies / Q000097:blood; D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D007453:Iodide Peroxidase / Q000276:immunology; D011247:Pregnancy / Q000097:blood*; D011446:Prospective Studies; D012016:Reference Values; D013961:Thyroid Gland / Q000276:immunology* / Q000503:physiopathology; D013963:Thyroid Hormones / Q000097:blood*; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D001323:Autoantibodies; D013963:Thyroid Hormones; C006606:anti-thyroglobulin; D014284:Triiodothyronine; D007453:Iodide Peroxidase; D013974:Thyroxine