Physiological changes necessitate the use of pregnancy-specific reference ranges for thyrotrophin (TSH) and free T4 (FT4) to diagnose thyroid dysfunction during pregnancy. Although many centers use fixed upper limits for TSH of 2.5 or 3.0 mU/L, this may lead to overdiagnosis or even overtreatment.

The new guidelines of the American Thyroid Association have considerably changed recommendations regarding thyroid function reference ranges in pregnancy accordingly. Any hospital or physician that is still using the 2.5 or 3.0 mU/l cut-off for TSH during pregnancy should evaluate their own lab-specific cut-offs.

The investigator's objective is to establish a rational reference range of serum TSH for the diagnosis of subclinical hypothyroidism in the first, second, and third trimester of pregnant women in the Sancaktepe region in Turkey.

Serum thyroid function tests (including T3, T4, TSH, Anti-TPO) will be obtained from the singleton pregnancies in the first, second, and third trimester to determine the local specific normal ranges.

Inclusion Criteria:

• Singleton pregnancy
• Free of pre-existing thyroid disease
• That do not use thyroid interfering medication
• That did not undergo IVF treatment
• TPOAb negative

Exclusion Criteria:

• Adolescent pregnancy
• Patient with pre-existing thyroid disease
• TPOAb positive
• Patient using thyroid interfering medication
• That had IVF treatment

Turkish pregnant women living in Sancaktepe Region of Turkey