Endocrine

Question 1

Fetal Thyroid Development (7)

Answer 1

• Thyroid develops from 3/40 from the primitive pharynx and migrates to the neck
• Starts producing thyroid hormone by 10-12 weeks
• Fetus is reliant on transplacental transfer of maternal T4 until fetal thyroid becomes active at 12/40
• Fetal T4 is detectable by 14/40
• Normal fetal and neonatal thyroid levels are crucial for brain maturation and intellectual development
• Fetus and fully breastfed infant are dependent on maternal iodine intake for thyroid hormone synthesis - supplementation is recommended

Question 2

Changes to Thyroid Function in Pregnancy (6)

Answer 2

• Gland hyperplasia with thyroid enlargement
• Increased GFR increases urinary iodine clearance so increased intake is required to make and maintain T4
• Oestrogen increases thyroxine-binding globulin synthesis and decreased clearance = increased T4/T3
• T4 increases/TSH decreases/TRH decreases due to circulating hCG
• Increased demand for T4 1-3% above daily needs
• T4 levels rise and plateau around 16-20 weeks

Question 2

Pregnancy-Specific Ranges for TFTs

TSH (3)
T4 (1)
Awanui TSH
Awanui T4

Answer 2

TSH:
Local ranges are recommended, if not available:
- 0.5mU/L less than non-pregnant range for 1st trimester
- Non-pregnant ranges for 2nd/3rd trimester
- 4mU/L is universally accepted as the upper limit of normal throughout pregnancy

T4:
- Method-specific ranges

Awanui Lab TSH:
- <7/40: 0.3-4.2mIU/L
- 7-<14/40: 0.1-3.5mIU/L
- 14-<28/40: 0.2-3.8mIU/L
- 28-40/40: 0.3-3.8mIU/L

Awanui Lab T4
- <7/40: 12-22pmol/L
- 7-<14/40: 12-21pmol/L
- 14-<28/40: 10-18pmol/L
- 28-40/40: 9-16pmol/L