Thyroid Hormone Synthesis Requirements
Iodine is essential for thyroid hormone synthesis, serving as a critical substrate for the production of thyroxine (T4) and triiodothyronine (T3). 1
Role of Iodine in Thyroid Hormone Production
Iodine functions as the fundamental building block for thyroid hormones:
- Three iodine molecules are required to form triiodothyronine (T3), while four are needed for thyroxine (T4) 2
- The thyroid gland stores 70-80% of the body's total 15-20 mg iodine content 1
- Daily thyroid hormone synthesis requires approximately 60-95 μg of iodine based on iodine turnover 1
The biochemical process of thyroid hormone synthesis involves:
- Iodination of the amino acid tyrosine 3
- Coupling of iodotyrosines:
- T4 is formed by coupling two molecules of diiodotyrosine (DIT)
- T3 is formed by coupling one molecule of DIT with one molecule of monoiodotyrosine (MIT) 3
Iodine Requirements and Recommendations
The World Health Organization (WHO) recommends:
- 150 μg/day for adults
- 220-250 μg/day for pregnant women
- 250-290 μg/day for lactating women 1
Consequences of Inadequate Iodine
Insufficient iodine intake leads to:
- Impaired thyroid hormone synthesis
- Hypothyroidism
- Goiter development
- Impaired neurodevelopment
- Increased risk of stillbirth and infant mortality
- Cognitive and neuromotor disabilities 1, 2
Iodine deficiency remains a significant global public health concern, with some countries including the UK listed among those with the lowest iodine status worldwide 1.
Iodine Excess and Thyroid Function
While iodine deficiency is problematic, excessive iodine intake can also disrupt thyroid function:
- Most healthy individuals tolerate high iodine intakes well
- However, excess iodine can precipitate hyperthyroidism, hypothyroidism, goiter, or thyroid autoimmunity in susceptible individuals 4
- People with preexisting thyroid disease or previous iodine deficiency are particularly vulnerable to thyroid dysfunction from excess iodine 4
Assessment of Iodine Status
Urinary iodine concentration (UIC) is the most reliable population indicator for assessing iodine status 1. Additional evaluations may include:
- Thyroid function tests (TSH, free T4)
- Thyroid size assessment 1
Clinical Implications
Understanding the essential role of iodine in thyroid hormone synthesis is crucial for:
- Proper diagnosis of thyroid disorders
- Appropriate supplementation in deficient populations
- Monitoring patients at risk of iodine-induced thyroid dysfunction
The answer to the question is clearly option A: iodine, as it is the fundamental substrate required for thyroid hormone synthesis, while ACTH, cortisol, and calcium are not directly involved in this process.