Thyroid disease in pregnancy

It is a Tuesday morning antenatal clinic. A 32-year-old with Hashimoto's disease walks in six weeks pregnant on 100 micrograms of levothyroxine, TSH 4.2 mIU/L. If you wait for the booking bloods at 12 weeks and react, you have left her hypothyroid through the window that matters. The fellowship question on thyroid disease in pregnancy is almost always a question about timing — when to dose, when to switch, when to refer, and when to do nothing at all.^[1][3]

Overview and definition

Thyroid disease in pregnancy is not one disease but four overlapping problems that share a remodelled endocrine axis. ^[1][2]

• Hypothyroidism — overt (TSH above the trimester reference with low free T4) or subclinical (TSH elevated with normal free T4) or isolated hypothyroxinaemia (low free T4, normal TSH). Most cases are autoimmune (Hashimoto's) or post-radioiodine.
• Hyperthyroidism — Graves disease (TSH-receptor antibody positive, the dangerous one), gestational transient thyrotoxicosis of hyperemesis (hCG-driven, antibody-negative, self-limiting), or less commonly toxic nodular disease.
• Postpartum thyroiditis — a triphasic autoimmune rebound in the year after delivery.
• Thyroid nodules and cancer — distinct clinical path; not the subject of this topic.^[1]

The unifying exam answer: pregnancy is a state of increased thyroid hormone demand, and every management decision — dose up, dose down, switch drug, screen, do not screen — flows from that single fact.^[3]

References18ShowHide

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2. [2]Stagnaro-Green A, Abalovich M, Alexander E, et al. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum Thyroid, 2011.PMID 21787128
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