The developmental window
Thyroid hormone drives neuronal migration, myelination and synapse formation — the physical construction of the brain. The fetal thyroid gland begins forming early but does not secrete useful quantities of hormone until roughly 16 to 18 weeks. Everything the brain does before that point runs on maternal T4 crossing the placenta.
Your own thyroid has to work harder to supply both of you. Demand rises by 30 to 50 percent from early pregnancy, driven partly by hCG, which weakly stimulates the thyroid, and partly by increased binding proteins. A thyroid that was borderline before pregnancy often cannot meet that increase.
What untreated hypothyroidism does
Overt, untreated maternal hypothyroidism in early pregnancy is associated with lower childhood IQ, delayed motor and language development, and a higher risk of miscarriage, pre-eclampsia, placental abruption, preterm birth and low birth weight. Severe iodine deficiency causes cretinism, with profound intellectual disability — historically common in the Himalayan and sub-Himalayan belt and the reason universal salt iodisation was introduced in India.
Subclinical hypothyroidism — a raised TSH with a normal T4 — is more common and more debated, but is generally treated in pregnancy, particularly when thyroid antibodies are positive.
Testing and targets
Ask for TSH and free T4 at your first visit, plus TPO antibodies if TSH is raised. Pregnancy reference ranges are lower than non-pregnant ranges — the commonly used first-trimester upper limit for TSH is around 2.5 mIU/L, and roughly 3.0 later, though laboratories increasingly use their own trimester-specific ranges. A TSH of 4 that would be ignored outside pregnancy is treated within it.
If you were already on levothyroxine before conceiving, your dose almost always needs increasing — often by 25 to 50 percent — as soon as pregnancy is confirmed. Do not wait for the next scheduled appointment.
Taking levothyroxine properly
- Same time every morning, on an empty stomach, 30 to 60 minutes before food.
- Keep iron and calcium tablets at least four hours away — they block absorption substantially.
- Do not stop it because you feel well. Feeling well is the point.
- Recheck TSH every four to six weeks through pregnancy, and again after any dose change.
Hyperthyroidism
Overactive thyroid in pregnancy is less common but also needs careful management, usually with propylthiouracil in the first trimester. Untreated, it raises the risk of preterm birth, pre-eclampsia and fetal growth restriction. Graves' antibodies can cross the placenta and affect the baby's thyroid, so specialist input is needed.
Iodine, practically
Iodine requirement rises to about 250 micrograms a day in pregnancy. Use iodised salt — check the packet, and store it in a closed container away from heat, as iodine degrades. Milk, curd and fish contribute. Do not take high-dose iodine or kelp supplements, which can paradoxically suppress the thyroid.
The short version
Test early, treat promptly, take the tablet correctly, recheck regularly. Managed thyroid disease in pregnancy has excellent outcomes; the harm comes almost entirely from going undetected through the first trimester.