**6.1 Free thyroxine (fT4) and free triiodothyronine (fT3)**

T4 is a molecule of thyronine bond to four atoms of iodine, while T3 has three iodine atoms. T4 is more abundant than T3, but through deiodinase activity, the more potent T3 is synthesised. Low level of T4 usually indicates hypothyroidism. For clinical biochemistry free T4 (fT4) and free T3 (fT3) are usually measured as these are the biologically active forms. Reference intervals for fT4 and fT3 are as stated in **Table 2**.

#### **6.2 Thyroid-stimulating hormone (TSH)**

TSH is released upon the induction by thyrotropin-releasing hormone (TRH), secreted from the hypothalamus. TRH is secreted when serum T3 and/or T4 is low. TSH induces the production of thyroid hormones T3 and T4. Low level of TSH usually indicates hyperthyroidism (parallel to high levels of T3 and/or T4) [17]. Reference interval for TSH is as stated in **Table 2**.

#### **6.3 Thyroglobulin (TG)**

TG is the globulin where binding of iodine to tyrosine to form thyroid hormones takes place. It is the long-term biomarker for iodine status in a human, besides urinary iodine as the short-term biomarker for iodine nutrition.

The median for reference interval for dried blood spot (DBS) TG from 5- to 14-year-old children before intervention of iodized salt is 49 g/L. After using iodized salt for 5 months, the DBS-TG decreased to 13 g/L and further decreases to 8 g/L after 10 months of consumption of iodized salt [20].

### **6.4 Thyroid antibodies [thyroid peroxidase antibodies (TPO Ab) and thyroglobulin antibodies (TgAb)]**

This antibody is for binding to the antigen thyroid peroxidase (TPO) enzyme which is responsible for thyroid hormones synthesis. Once the enzyme is bound to the antibody, less free unbound enzymes are available for thyroid hormone

#### *Biochemical Testing - Clinical correlation and Diagnosis*

**Figure 4.** *Subsequent biomarker testing and further action for individual and population urinary iodine estimation.*

synthesis, thus causing lower thyroid hormone production. TgAb is the antibody for the globulin TG. With the presence of elevated amount of TgAb, the thyroglobulin (TG) will bind to its antibody, and lesser TG is available to bind to the thyroid hormones for transportation in the blood vessels, thus causing lesser thyroid hormones being circulated in the human body [21]. The reference intervals are 15 kIU/L for TPOAb and 31 kIU/L for TgAb [22] (**Figure 4**).
