Thyroid Hormone Synthesis Requires Iodine
Iodine is essential for thyroid hormone synthesis, making option A (iodine) the correct answer. 1
Role of Iodine in Thyroid Hormone Production
Iodine serves as a critical substrate for the synthesis of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Without adequate iodine, proper thyroid hormone production cannot occur, leading to significant health consequences:
- Three iodine molecules are incorporated to form triiodothyronine (T3)
- Four iodine molecules are needed to produce 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
Why Other Options Are Incorrect
Option B (ACTH): Adrenocorticotropic hormone stimulates cortisol production in the adrenal glands but has no direct role in thyroid hormone synthesis.
Option C (Cortisol): Cortisol is a steroid hormone produced by the adrenal glands. While it can influence thyroid function indirectly through feedback mechanisms, it is not required for thyroid hormone synthesis.
Option D (Calcium): Calcium is important for many physiological processes but is not directly involved in the synthesis of thyroid hormones. Calcium homeostasis is regulated in part by parathyroid hormone, which is distinct from thyroid hormone production.
Biochemical Process of Thyroid Hormone Synthesis
The synthesis of thyroid hormones involves several iodine-dependent steps:
- Iodide (I-) is actively transported into thyroid follicular cells via the sodium-iodide symporter
- Iodide is oxidized to iodine (I2) by thyroid peroxidase
- Iodine attaches to tyrosine residues in thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT)
- T3 is formed by coupling one MIT and one DIT molecule
- T4 is formed by coupling two DIT molecules 2
Clinical Implications of Iodine Deficiency
Inadequate iodine intake leads to impaired thyroid hormone synthesis with serious consequences:
- Hypothyroidism
- Goiter (thyroid enlargement)
- Impaired neurodevelopment, particularly in fetuses and infants
- Increased risk of stillbirth and infant mortality
- Cognitive and neuromotor disabilities 1, 2
Recommended Iodine Intake
Various health organizations have established recommended daily iodine intake levels:
- Adults: 150 μg/day (WHO/FAO)
- Pregnant women: 220-250 μg/day
- Lactating women: 250-290 μg/day 1
Monitoring Iodine Status
Iodine status can be assessed through:
- Urinary iodine concentration (UIC) - the most reliable population indicator
- Thyroid function tests (TSH, free T4)
- Thyroid size assessment 1
Iodine deficiency remains a significant public health concern globally, with the UK listed among the ten countries with the lowest iodine status worldwide 1.