*1.2.1 Thyroid hormone synthesis*

*Biochemical Testing - Clinical correlation and Diagnosis*

*determine the median urinary iodine concentration (mUIC).*

The main biochemical indicator that is widely used for the assessment of IDD is urinary iodine concentration (mUIC) [4]. The advantages of mUIC as an indicator of IDD are that the method directly reflects iodine supply of the individual, it is objective and non-invasive and urine samples can be kept for later analysis. However, the disadvantages of this method are that it requires laboratory space, special facilities and skilled technician to provide accurate determinations. In addi-

*Degrees of iodine intake (iodine nutrition status) and their suitable types of human sample collection to* 

Epidemiological studies stated that the population distribution of urinary iodine

is required rather than individual levels. The frequency distribution of urinary iodine usually skewed towards elevated values; hence, the median value is considered instead of the mean as indicating the status of iodine nutrition [1]. The mUIC of 100 μg/L and above defines a population which has no IDD; i.e. at least 50% of the sample should be >100 ug/L. In addition, not more than 20% of sample should be below 50 μg/L. Iodine nutrition status is based on six categories of urinary excre-

tion, this method reflects only current but not past intake of iodine [5].

**Median urinary iodine concentration (μg/L) Severity of IDD** <20 Severe deficient 20–49 Moderate deficient 50–99 Mild deficient 100–199 Optimal

200–299 More than adequate

*Epidemiological criteria for assessing IDD in a population based on median urinary iodine concentration.*

>300 Excessive

**14**

**Table 1.**

**Figure 1.**

*Source: Ref. [3].*

tion classification (**Table 1**) [3].

Iodine is grouped under micronutrients, and it is needed in small amount, but it is very important for the development of optimum human growth. Iodine is needed in the synthesis of thyroid hormones [6]. Through iodination, one, two, three or four iodine atoms are bound to tyrosine to form monoiodothyronine (MIT), diiodothyronine (DIT), triiodothyronine (T3) or thyroxine (T4), respectively, through the action of iodinase enzyme. Iodine is absorbed from the gastrointestinal system, will enter the blood circulation and will be transported into the thyroid follicle cells through the sodium/iodine (Na/I) symporter. In iodide form, it will then be transported to the thyroid follicle colloid through pendrin. Concurrently, thyroglobulin (TG) is being synthesised in the endoplasmic reticulum (ER) and being secreted into the follicle colloid through exocytosis. TG is the transporter protein of the thyroid hormones in the thyroid follicle colloid. It consists of branches of tyrosine molecules which will then be bound to iodine through the iodination process, forming the MIT and the DIT. When one MIT and one DIT bind, T3 will be formed, while upon binding of two DITs, T4 will then be formed. These TG-bound thyroid hormones will enter the thyroid follicular cell through endocytosis. TG will then undergo proteolysis, and T3 and T4 will be transported into the blood circulation through the MCT symporter [7] (**Figure 2**).
