Thyroid Function and Iodine Status in Pregnancy

Thyroid hormones are essential regulators of maternal metabolism and foetal neurodevelopment. During pregnancy, rising levels of human chorionic gonadotrophin and increased renal clearance stimulate the maternal thyroid gland to produce up to 50 per cent more thyroxine. This augmented demand requires higher iodine intake, as iodine is the key substrate for thyroid hormone synthesis. Insufficient iodine availability can impair maternal thyroid hormone production, leading to suboptimal transfer of thyroxine to the developing foetus, especially in the first trimester when the foetal thyroid is not fully functional. Conversely, excessive iodine intake may provoke transient hypothyroidism or autoimmune thyroiditis. Public health measures such as universal salt iodisation and targeted supplementation aim to maintain iodine sufficiency in pregnant women, yet geographic variability in iodine content of water, soil and food remains a challenge. Screening strategies for thyroid dysfunction in pregnancy balance the benefits of early detection against the risks of unnecessary treatment, while ensuring that both deficiency and excess are minimised to safeguard maternal health and optimise childhood developmental outcomes.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Research from all publishers

Recent cohort work in infertile women with subclinical hypothyroidism has shown that serum thyroid-stimulating hormone setpoints recorded before conception remain largely stable throughout gestation, while levels of thyroid peroxidase antibodies decline as pregnancy advances. This suggests that treatment thresholds established preconception may reliably guide thyroxine supplementation, and that monitoring free thyroxine could offer greater sensitivity than TSH in early pregnancy.

A systematic review and meta-analysis of milk-iodine concentration across 34 countries has revealed significant seasonal and farming-system variations. Winter milk contains almost 6 μg more iodine per 100 g than summer milk, and conventional dairy yields higher iodine content than organic. Such fluctuations underscore the importance of accounting for dietary sources of iodine when assessing population iodine status and planning nutritional surveys for pregnant women.

A meta-analysis of individual participant data from three European birth cohorts explored maternal urinary iodine/creatinine ratio and child IQ. A positive association with verbal IQ was detected, primarily when iodine status was assessed before 14 weeks’ gestation. These findings highlight the critical window of early pregnancy for iodine sufficiency and support the case for timely supplementation in regions of mild to moderate deficiency.

Thyroid Function and Iodine Status in Pregnancy publication trend

The graph below shows the total number of articles in thyroid function and iodine status in pregnancy across all publications each year (not limited to Nature Index journals).

Technical terms

Thyroid-stimulating hormone (TSH): A pituitary hormone that regulates thyroid gland activity, commonly used to assess thyroid function.

Free thyroxine (fT4): The unbound fraction of the hormone thyroxine in blood, reflecting the metabolically active thyroid hormone level.

Urinary iodine concentration (UIC): The amount of iodine excreted in urine, expressed in μg/L, serving as an indicator of recent iodine intake.

Urinary iodine/creatinine ratio (UI/Creat): A correction of UIC for urine dilution, calculated in μg/g, providing a more stable measure of iodine status.

Subclinical hypothyroidism: A condition characterised by elevated TSH with thyroid hormone levels within reference ranges, often asymptomatic but monitored in pregnancy.

References

1. In infertile women with subclinical hypothyroidism, with or without thyroid peroxidase antibodies, serum TSH during pregnancy follows preconception values and thyroid hormones remain stable. Human Reproduction Open (2023).
2. Variation in milk‑iodine concentration around the world: a systematic review and meta-analysis of the difference between season and dairy-production system. Food Chemistry (2024).
3. Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data. The Journal of Clinical Endocrinology & Metabolism (2019).
4. Effect of excess iodine intake on thyroid diseases in different populations: A systematic review and meta-analyses including observational studies. PLOS ONE (2017).
5. GLOBAL ENDOCRINOLOGY: Global perspectives in endocrinology: coverage of iodized salt programs and iodine status in 2020. European Journal of Endocrinology (2021).