This is education, not medical advice, and nothing here is a diagnosis or a promise about your fertility. Every number below is a conversation to have with your own provider — read as part of a bigger picture, not acted on alone.
Hormone signaling is the messaging between your brain, thyroid and ovaries that governs whether — and how well — you ovulate and implant. "Normal" hormone labs can still miss what's happening, because a reference range is the band where about 95% of a tested population falls, not a target set for conception. The markers most often misread are your thyroid (TSH, and the antibody almost no one checks), your mid-luteal progesterone (where timing is everything), and the ratios and outliers like LH:FSH and prolactin. What matters isn't any single value — it's how they read together.
If your fertility hormones came back "all normal" and you're still waiting, this is the driver most likely to feel like a dead end. The panel looks fine. The doctor moves on. And you're left with a page of numbers and no interpretation of what they mean for your chance of conceiving.
Here's the shift that changes everything: hormones don't work as a list of independent values. They work as a signaling system — a cascade of messages, each one shaping the next. Reading them one flag at a time is how the important things slip through. Let's read them as a system.
What is "hormone signaling," and why does it decide conception?
Ovulation isn't a single event — it's the end of a conversation. Your hypothalamus signals your pituitary, which releases FSH and LH; those tell the ovary to mature and release an egg; after ovulation, the ovary produces progesterone to prepare the lining for implantation. Sitting over all of it, the thyroid quietly sets the tempo. When any part of this messaging is slightly off, the labs can still land "in range" while the cadence — timing, strength, follow-through — is subtly wrong.
That's why "normal" and "optimal for conception" are different questions. A range built to describe a general population was never calibrated to ask what level gives this woman the best chance of conceiving? For several of these markers, the conception-relevant answer is narrower than the lab's flag.
Your thyroid: the number, and the antibody nobody checks
What should my TSH be if I'm trying to conceive?
TSH — thyroid-stimulating hormone — is the single most useful thyroid number, and the thyroid sets the pace for ovulation and early pregnancy. Many labs flag TSH as "normal" up to roughly 4–4.5 mIU/L. But the American Thyroid Association and the Endocrine Society have suggested a preconception target below about 2.5 mIU/L for women who are hypothyroid or trying to conceive, because higher values have been associated with more adverse reproductive outcomes.
This is genuinely debated. The American Society for Reproductive Medicine's 2024 guidance concluded there is insufficient evidence that a TSH between 2.5 and 4 — on its own, without thyroid antibodies — worsens outcomes in the general population trying to conceive. So the right reading isn't "your 3.2 is the reason." It's: a TSH in the high-2s or 3s is worth interpreting alongside your antibodies, not waved through as "fine."
The antibody that matters even when your TSH is normal
Here is the marker most often missing from the picture: thyroid peroxidase antibodies (TPO antibodies). You can have a perfectly "normal" TSH and still carry thyroid autoimmunity — and it leaves a measurable footprint. Across a large body of research in women without overt thyroid disease, antibody positivity has been associated with a meaningfully higher risk of miscarriage (in one widely-cited synthesis, roughly a 2-to-4-fold increase), and with lower fertility in the subgroups of women with recurrent loss or infertility.
Two caveats, because this is where interpretation matters most. First, this is an association — antibodies mark risk; they don't prove cause in any individual. Second, and importantly: large trials have not shown that adding thyroid hormone (levothyroxine) to antibody-positive women with normal TSH improves live-birth rates. So the value of the TPO test isn't a treatment you demand — it's information that changes how closely your thyroid is watched, especially in early pregnancy, where a meaningful share of antibody-positive women drift into subclinical hypothyroidism. Ask for it; interpret it with your provider.
Progesterone: the hormone where timing is everything
Progesterone is what your ovary makes after ovulation. A well-timed level does two useful things: it confirms you actually ovulated, and it hints at how strong that ovulation was. The catch — and the reason so many progesterone results are misread — is timing. Progesterone peaks about seven days after ovulation: roughly cycle day 21 in a textbook 28-day cycle, but later if you ovulate later. A mid-luteal level above about 10 ng/mL is commonly used to confirm ovulation occurred.
"Low progesterone" is not a clean diagnosis. ASRM's committee opinion concludes there is no validated, standardized test for luteal phase deficiency, and it is not established as an independent cause of infertility. So the highest-value insight here isn't a number to fear — it's timing. A progesterone drawn on "day 21" when you actually ovulated on day 18 or day 24 can read falsely low. If your cycles aren't 28 days, the standard test is often simply mistimed — which is an interpretation problem, not a hormone problem.
The ratios and the outliers: LH:FSH and prolactin
Two more signals are worth interpreting rather than glancing at. The LH:FSH ratio can hint at the pattern seen in PCOS, where LH runs relatively high — though PCOS is diagnosed on a combination of features, not a single ratio, and it overlaps heavily with the metabolic driver. And prolactin, a hormone better known for milk production, can — when elevated — suppress the ovulation signal entirely, which is exactly the kind of thing a standard "fertility panel" may not include unless someone thinks to ask.
Neither is a number to panic over in isolation. Both are examples of the same principle: the answer lives in the relationships between hormones, not in any one flag.
When your cycle is itself the signal
Your period is a monthly readout of the entire signaling system — arguably the most information-dense fertility marker you already have, and it's free. A cycle that runs consistently 24–35 days, with a luteal phase of at least ~10 days, suggests the cascade is completing. Cycles that are long, short, unpredictable or absent are telling you the messaging is breaking down somewhere: ovulation may be delayed, weak, or not happening at all.
This is why "how long are your cycles, and how regular?" is often more revealing than a single hormone draw. A short luteal phase points toward the progesterone story above. Long or erratic cycles point toward the ovulation trigger — and frequently toward the metabolic driver, where insulin patterns nudge the LH signal. The cycle doesn't replace the labs; it tells you which labs, drawn on which day, will actually mean something.
Why hormone signaling can't be read alone
Here's what turns a list of numbers back into a system. Your thyroid — the pacemaker of this whole driver — is built and run by nutrients from another driver entirely. Iron is a structural component of thyroid peroxidase, the enzyme that makes thyroid hormone; iron deficiency can lower circulating thyroid hormone and blunt the conversion of the storage form (T4) into the active form (T3). Selenium runs that same T4-to-T3 conversion through the deiodinase enzymes. So a TSH that reads "a little off" may not be a thyroid problem at all — it may be a nutrient problem wearing a thyroid mask.
That is the whole case for interpretation over single numbers. A hormone value is a symptom of the system around it. Reading Hormone Signaling in isolation — without the nutrient status feeding it or the metabolic patterns pushing it — is exactly how the real story gets missed.
So what should I actually do with all this?
Not demand a stack of tests. The move is a sharper, calmer request — and then an interpretation. You might ask your provider some version of:
"Can we look at TSH together with thyroid (TPO) antibodies, a correctly-timed progesterone, and — if my cycles are irregular — prolactin and the LH:FSH pattern? And where they're 'in range,' can we read them as a system, against conception-relevant targets, rather than one flag at a time?"
That single reframe is the whole point of this driver. It moves you from "your hormones are normal, keep trying" to a read of how your signaling is actually behaving — which one is leading your picture, and what to do first. None of this is a guarantee, and none of it replaces your clinician. But it is the difference between being dismissed and being interpreted.