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TSH and ST4 tests should be the first tests to be conducted in the evaluation of thyroid functions.
According to TSH upper limit age;
4 mIU/L in healthy young population
2,5 mIU/L in those contemplating pregnancy and pregnant
6 mIU/L between 70-79 years of age
7,5 mIU/L over 80 years of age

Thyroid Tests
TSH measurement is sufficient as thyroid screening test.
Upper limit of TSH needs to be considered as 5 mlU/L.
Upper limit of TSH should be targeted as 2.5 mIU/L in women contemplating pregnancy.
sT4 and/or total T4 should be consulted in pregnancy. Since sT4 is open to measurement errors in pregnancy, total T4 measurement should be preferred.
TSH, free T4 and total T3 (or free T3) should be minimum investigated in patients strongly suspected of availability of thyroid disease or with thyroid disease in clinics and laboratories. 
TSH should be used in follow-up of primary hypothyroidism. Monitoring of patients to whom thyroid hormone replacement has been performed should be regulated with TSH measurements conducted once 4-6 weeks.
Free T4 should be measured in follow-up of secondary hypothyroidism. TSH measurements are not useful in follow-up process while performing secondary hypothyroidism replacement.

Thyroid Auto Antibodies  
Cases in which thyroid antibody measurement is required:

Measurement should be conducted in the persons with TSH > 4 mIU/L. 

Measurement of serum anti-TPO and anti-Tg levels enables diagnosis of chronic autoimmune thyroiditis. Low anti-Tge titres can be found in the elderly persons and those with another autoimmune disease. Since anti-Tg is found as high in almost all patients with autoimmune thyroid disease and anti-TPO positivity, this antibody do not contribute much to this antibody diagnosis. Anti-TPO and anti-Tg is positive in the rate of 95-100 % in autoimmune thyroiditis (Hashimoto's thyroiditis) and positive in the rate of 60-90% in Basedow-Graves disease. Low titres may be found in normal population and non-autoimmune thyroid patients. Diagnosing anti-TPO in pregnancy is associated with increased risk of miscarriage even in euthyroid individuals. For these reasons, threshold of starting thyroid hormone treatment of antibody positivity in pregnancy is lower.  There is no conclusive view relating to administering T4 to pregnant women with Anti-TPO positive and TSH <2.5 mIU/L.

Cases in which thyroid autoantibody measurements are used
 • Post-partum thyroiditis
 • This is a precursor of thyroid insufficiency in subclinic hypothyroidism.
 • Anti-TPO high level is a precursor of the fact that some drugs may cause thyroid dysfunction (amiodarone, interferon, IL-2, lithium).
 • Thyroid autoantibody positivity is a risk factor for IVF unresponsiveness.
 • Neonatal hypothyroidism risk factor
 • Very high Anti-TPO titre in thyroid lymphoma 
 • Anti-TPO measurement is sufficient for diagnosis of autoimmune thyroid disease.
 • Anti-Tg measurement should be conducted in above-mentioned special cases. (Tg follow-up in thyroid cancer, anti-TPO negative autoimmune thyroid diseases).

Thyroglobulin Measurement
Thyroglobulin measurement is used for two main reasons.
1 - Differentiated thyroid cancer follow-up 
2 - Hyperthyroidism "thyrotoxicosis Factitia" depending on external thyroid hormone intake.

TSH Receptor Antibody
TSH receptor antibodies (TSHRAb)
Sometimes in difficult cases where Graves' disease can be diagnosed, even if these antibodies are resorted; generally clinical findings are sufficient to diagnose.

Thyroid Stimulating Hormone Antibody Measurement Indications
These measurements can be used for 
 • Exophthalmos (eye protrusiveness) differential diagnosis (euthyroid Graves' disease, unilateral exophthalmos)
 • Nodule Graves and nodular toxic goiter differential diagnosis (decision to treat)
 • Non-autoimmune thyrotoxicosis differential diagnosis
 • Graves' disease in pregnancy (in terms of neonatal thyrotoxicosis risk) deciding on Graves' disease remission following antithyroid  treatment. 

TSHRAb examined in early period when Graves' disease occurs in pregnancy, it is helpful to distinguish from gestational thyrotoxicosis. After 20 weeks, this antibody passes to the foetus. If TSHRAb is present in the blood in high levels as of twenty-eight weeks, this is stimulating in terms of neonatal thyrotoxicosis. If Graves' disease has been already treated with radioactive iodine or surgically in mother, TSHRAb measurement should be requested in 20th-24th week and in the first trimester of pregnancy.

Thyroglobulin measurement should be used in differential diagnosis of "thyrotoxicosis Factitia" and follow-up of differentiated thyroid cancer. Thyroglobin measurement has no diagnostic value in other thyroid diseases.

Measurements In Pregnancy 
Very important changes are experienced in thyroid physiology during pregnancy. TBG increases during this period and therefore T3 and T4 levels depending on protein are increasing as well. While free T4 levels remain within normal limits in the first trimester of pregnancy, this can slightly increase and TSH level is very low (<0,1 mIU / L) Human chorionic gonadotropin increasing in this period shows no thyrotropic effect. In following trimester, TSH levels come to normal levels.

Thyroid Function Tests in Pregnancy and Postpartum
Hypothyroid in mother in the first trimester plays an adverse role in psychomotor development of fetus.  For this reason, it should be less than TSH 2.5 mIU/L and should be less than 3 mIU/L in the second and third trimesters. Since there is a risk of developing hypothyroidism in women with thyroid autoimmunity, TSH should be monitored closely. T4 rises as 1.5 times since TGB increases in the rate of 60-80 % as of 20th week of pregnancy. Reference ranges given by the manufacturer of the free T4 values are not valid during pregnancy. In recent times, trimester specific free T4 values ​​have been published. 

Normal reference values are not used for TSH during pregnancy. Because human chorionic gonadotropin causes suppression of TSH levels especially in the first trimester human. For this reason, suppressed TSH can be inaccurately assumed as subclinical hyperthyroidism or slight high TSH can be inaccurately perceived as “normal". 

Trimester-specific TSH values ​​have been determined. It has been reported that normal lower limit of TSH in the first trimester is 0.1 mIU/L and normal lower limit of TSH is 0.2 and 0.3 mIU/l in second and third trimester. TSH may even be 0.01 mIU/l in some normal pregnancies. TSH values over 2.5 mIU/L in the first trimesters and 3.0 mIU/L in the second and third trimesters should be evaluated as subclinical hypothyroidism. Lymphocytic hypophysitis caused by rare hypothyroidism in pregnancy and in the postpartum period may lead to lower TSH. Hyperemesis gravidarum should be considered in women with severe vomiting in early pregnancy, losing 5% of weight and ketosis and dehydration. In this period, TSH has already been suppressed TSH below the normal limits. However, free thyroid hormones have risen in the rate of 30-60 % of women with hyperemesis. However, free T3 index is considered normal. Most of these women have no symptoms of hyperthyroidism. Stimulating antibodies of TSH receptor should be examined for distinguishing it from Graves' disease. Women with Anti-TPO positivity in the first trimester of pregnancy should be monitored after 3 and 6 months after birth in terms of postpartum thyroiditis. 

Conducting thyroid screening of all pregnant women is not accepted in our time. Yet, screening those who are accepted as risk group is appropriate. Only pregnant women with high risk should be screened for thyroid disease.