What should my thyroid levels be if I'm trying to conceive?

Thyroid testing is one of the few places where mainstream and functional medicine openly disagree about the same blood test. Here's what each says, and where the evidence actually sits.

The numbers

The last row is where the famous "under 2.5" number comes from. It's a target for adjusting medication in someone already being treated — not a threshold for starting it in someone who isn't.

Moderate evidence If your TSH is between 2.5 and 4

You will find a great deal written about this range. ASRM's 2024 guideline, which reviewed the evidence systematically, concluded two things: a TSH in this band isn't associated with a higher risk of miscarriage, and treating it with levothyroxine hasn't been shown to reduce loss. Both conclusions were rated moderate strength.

That doesn't mean the number is meaningless, or that a doctor who treats it is wrong to consider it. It means the evidence doesn't currently support treating the number by itself, and that an unnecessary thyroid medication has its own downsides.

Moderate evidence Thyroid antibodies

Thyroid peroxidase (TPO) antibodies are common, and women who have them do miscarry more often — that association is real. The question is whether treating helps.

The TABLET trial randomized women with normal thyroid function, positive TPO antibodies and a history of miscarriage or infertility to levothyroxine or placebo before conception. It did not increase live births.

ASRM doesn't recommend routinely screening for thyroid antibodies in infertility, though it says targeted testing can be considered after recurrent loss. If your antibodies are positive, the useful conversation is about monitoring your thyroid function closely in pregnancy — not about starting a medication the trial found didn't help.

What's worth asking for

• TSH, before you conceive if you can. If it's abnormal, free T4 comes next.
• A retest in pregnancy if you're on levothyroxine, because the dose almost always needs to go up, often by a lot and early.
• TPO antibodies if you've had recurrent loss, or a thyroid condition in the family — knowing changes how closely you're watched.

Questions to ask

"What's my actual TSH number, and what's this lab's normal range?" "If you'd treat me at 3.0, what's the evidence you're going on?" "If I'm already on levothyroxine, how soon after a positive test should we recheck it?"