Subclinical, and the pregnancy exception

Two situations sit at opposite ends of the 'when to treat' question. A mildly raised TSH in an older adult often needs watching, not treating. But in pregnancy — and when trying to conceive — the threshold drops sharply, and getting it right genuinely matters for the baby.

Subclinical: treat or watch?

The grey zone: Subclinical hypothyroidism means a raised TSH with still-normal T4. Above 10, most people are treated. Between about 4.5 and 10 it's individualised — a trial of treatment makes sense with symptoms, positive antibodies (i.e. Hashimoto's, which progresses faster), or plans for pregnancy; otherwise watch and repeat. The landmark TRUST trial found that treating mild cases in older adults didn't make them feel any better, so don't reflexively medicate a borderline number. Evidence ●●●●○

Pregnancy: the non-negotiable

Tighter targets, higher stakes: Pregnancy flips the calculus. Even antibody-positive women with 'normal' function have higher miscarriage and preterm risk, and the developing baby depends on the mother's thyroid hormone early on. Targets tighten (often aiming for TSH under 2.5), levothyroxine needs typically rise 20–30% as soon as pregnancy is confirmed, and testing gets frequent. Screen and optimise before conceiving. Evidence ●●●●○

Pregnant or trying? Tell your doctor early

If you have Hashimoto's (or are antibody-positive) and are pregnant or trying, flag it early: thyroid targets are tighter in pregnancy and your dose usually needs to go up promptly. This is the one scenario where acting fast clearly protects both you and the baby.

The Hashimoto’s Guide