Subclinical hypothyroidism and fertility: 5 subtle signs your thyroid is affecting your ability to conceive (even with normal test results)

Trying to conceive can sometimes feel like a puzzle with missing pieces that you just can’t work out what is missing. You track your cycles; you do everything right, yet month after month the positive test does not appear.  

When we think about thyroid health, we usually picture the big, obvious red flags, which is sudden weight gain, hair loss, extreme fatigue, constipation, or feeling the cold. Because these are the symptoms most commonly associated with an underactive thyroid, it is easy to assume your thyroid is functioning perfectly if you don't have these symptoms.

I know this frustration firsthand. In my own 10-year journey with secondary infertility, thyroid issues contributed to my recurrent chemical pregnancies, yet I didn't have typical symptoms like weight gain or constipation, and my energy was good. The only clue that there might have been something up with my thyroid, was a consistent low basal body temperature (BBT), which averaged around 35.7 in the follicular phase (day 2 – 10) of my cycle (the normal range is 36.4 - 37.0). I frequently raised this with my GP and fertility clinic, even at the hospital at my post D & C check-in after a 6-week miscarriage, but they all dismissed it as unrelated at the time. Back then, I couldn't order the comprehensive thyroid panel testing that I desperately needed; I had to rely on standard TSH and Free T4 tests, which always came back "normal." But my gut told me there was more to the story, especially when my TSH was often really low (less than 0.5)

The thyroid is the "master of metabolism" for every cell in your body, which means that it controls your metabolism and how your body uses energy. A thyroid dysfunction can be very subtle and can often go unnoticed. Often, symptoms are blamed on stress or a random cycle, when the thyroid is the actual culprit, quietly working below the surface.

If you’re struggling to conceive, here are five subtle signs of subclinical hypothyroidism that may be quietly affecting your fertility.

1. Short luteal phase and low progesterone (thyroid-related fertility sign).

"Progesterone is the hormone that holds your luteal phase together, and T3 is what helps progesterone rise."

The luteal phase is the crucial window of time between ovulation and your period. During this time, your body relies on the hormone progesterone to sustain a healthy uterine lining. A luteal phase under 10 days is often a sign of low progesterone.

T3, the active thyroid hormone, directly stimulates progesterone production by binding to receptor sites that are on the ovarian luteal cells. This encourages these cells to produce more progesterone. When T3 is low, progesterone drops too early, shortening the luteal phase,  which is otherwise known as luteal phase defect.

Low T3 can be caused by:

• Poor T4 → T3 conversion in the liver

• Low zinc, selenium, iron and vitamin D (required for T4 → T3 conversion)

• High inactive reverse T3 from stress or inflammation or excessive dieting (reverse T3 binds to T3 receptor sites in the ovaries, blocking active T3)

2. Subclinical anovulation (even with regular periods)

"A regular period doesn’t guarantee ovulation, and thyroid dysfunction is one of the most common reasons for silent anovulatory cycles."

Although you may get a period each month, this doesn’t mean you have definitely ovulated, as a regular monthly period doesn't always guarantee ovulation. I wish I had known this at the time, as I always assumed that a period meant you ovulated. Thyroid imbalances can disrupt the communication between your brain and ovaries (the HPO axis), leading to cycles where you bleed but don’t release an egg, which is known as an anovulatory cycle. Without ovulation, conception is impossible, even if your cycle looks and feels normal.

Signs of anovulation include:

• No temperature shift

• Negative ovulation tests (reflects no LH surge)

• Low day‑21 progesterone blood test (if you don’t ovulate your progesterone is low)

3. Low basal body temperature (BBT) in the follicular phase

"Your basal body temperature is a window into your metabolism, and your metabolism is powered by your thyroid."

Thyroid hormones are your internal thermostat. If you take your BBT upon waking between day 2 and 10 of your cycle and it is consistently below 36.0°C, it is a strong indication that your cellular metabolism may be running a little slow to support the energy-intensive process of maturing and releasing a quality egg.

Your BBT is your basal body temperature at rest, measured right after you wake up in the morning before you get up and move. It directly ties to your basal metabolic rate (BMR), the minimum amount of energy your body requires to function. When you have low thyroid function (in particular low T3), your metabolism slows down to protect itself, and your baseline internal temperature drops. Because your calls burn less energy and run at a slower pace, they produce less heat as a by-product, which is why it is easier to feel the cold if you have a low thyroid.

This was my biggest clue that my thyroid wasn’t working optimally. I frequently took my BBT upon waking from about day 2 to day 14 of my cycle, and it would always average around 35.5 - 35.9. In the luteal phase, it averaged around 36.0 to 36.2 which was higher than in the follicular phase, but still low, as it should be around 37.0 as progesterone has a natural thermogenic effect, raising body temperature in the luteal phase. To me, my temperatures seemed really low, so I questioned it a few times with my doctor and fertility clinic, but they didn’t seem overly concerned as my TSH was normal (or so they thought). This went on for several years as I tried my hardest to balance my thyroid naturally. Nowadays, if any of my clients have a low BBT, I will run a full thyroid panel to check their T3 levels.

4. Breast tenderness & fluid retention (thyroid-driven prolactin changes)

"When thyroid hormones drop, prolactin often increases, which is one of the fastest ways your body can shut down ovulation."

When the thyroid is sluggish because of low levels of T3 and T4, the brain releases higher levels of thyrotropin-releasing hormone (TRH) to compensate. This stimulates the production of prolactin from the pituitary gland, which signals to your body that it is not a suitable time for pregnancy.

Elevated prolactin can:

• Delay ovulation

• Prevent ovulation

• Cause breast tenderness

• Increase fluid retention

Even a mild elevation in prolactin can interfere with fertility.  Since thyroid function influences high prolactin levels, the first step is to address the root cause of the thyroid problem.  Nutrients such as vitamin B6, zinc, magnesium, and the herb vitex (Chaste Tree) may also be helpful in regulating prolactin, but it's best to work with a qualified practitioner.

5. Recurrent chemical pregnancies and thyroid autoimmunity

"Thyroid antibodies can interfere with implantation long before your thyroid hormones ever look abnormal."

Sometimes the challenge isn’t conceiving but maintaining the pregnancy.  Thyroid autoimmunity, with the presence of TPOAB (Hashimoto's Thyroiditis) and TgAb (Graves' Disease) antibodies, can disrupt implantation even when TSH, Free T4, and Free T3 appear normal in a blood test. These antibodies create inflammation in the uterine lining, reduce pinopodes, alter receptivity markers, and shift immune cell balance, all of which increase the risk of early pregnancy loss.

Even if thyroid antibody levels are not especially elevated, these antibodies can increase the risk of very early implantation failure by triggering a maternal immune response, which interferes with early placental development. The presence of thyroid antibodies indicates that there may be a highly reactive, over-vigilant maternal immune system that is causing the pregnancy to be rejected.

For successful implantation, an embryo requires the mother's immune system to enter a state of immune tolerance, meaning it must intentionally dial down its defences to avoid attacking the embryo. Elevated thyroid antibodies indicate that the immune system is in a heightened, auto-reactive state. Reproductive immunologists believe the presence of these antibodies may interfere with the delicate communication between the embryo and the uterine lining, causing the maternal immune system to treat the newly implanted embryo as a foreign threat rather than a welcome guest. As a result, the immune system attacks the embryo, resulting in a chemical pregnancy. If you are experiencing recurrent early chemical pregnancies that occur between 4 and 5 weeks, then this is a sign of recurrent implantation failure which could be to do with the presence of autoimmune antibodies, so this needs to be investigated. As many as 23% of women with unexplained infertility have thyroid antibodies.

Why standard thyroid tests miss so much

"Normal TSH doesn’t mean normal thyroid function; it only tells you what the pituitary is doing, not what your cells are experiencing."

Most doctors only test TSH, which is not enough for a fertility assessment.  While TSH is a signal from the pituitary gland to the thyroid gland, it doesn't truly reflect how your cells are actually utilising thyroid hormone, for this we need to monitor free T4, free T3 and reverse T3. Here is a summary of the optimal thyroid ranges.

Optimal fertility ranges:

• TSH: 0.5–2.0

• Free T4: 15–23

• Free T3: 5–7

• Reverse T3: 11–18

• TPO/Tg antibodies: ideally <2

Many women have “normal” results but still experience symptoms because:

• T4 isn’t converting to T3

• Reverse T3 is blocking thyroid receptors so T3 can get there

• Antibodies are silently attacking the thyroid

• The pituitary isn’t sending strong signals to the thyroid (low TSH)

TSH is a pituitary hormone that stimulates the thyroid gland to produce T4 when needed. Doctors consider your thyroid normal if your TSH falls within the standard TSH reference range of 0.4–4.0 mIU/L. For fertility, we look for an optimal TSH between 0.5 and 2.0 mIU/L. Anything above 2.5 mIU/L often suggests suboptimal thyroid function (hypothyroidism).

The problem is many people have normal TSH levels and still have symptoms of low thyroid function. Unfortunately, in these cases testing just TSH highlights what the pituitary gland is doing, and not the actual function of the thyroid. If your TSH is lower or higher than the reference range of 0.4 – 4.0 mIU/L , then your doctor will likely test your free T4 (thyroxine) which is used to find out how well your thyroid is functioning. The standard reference range for free T4 is 9 – 23 pmol/L so if your free T4 result is, say, 10 or 12 (which is what I see commonly) your thyroid function is considered to be normal.

An optimal Free T4 range is 15–23 pmol/L. Many women I see have results around 10 or 12, which are technically "normal" but indicate a sluggish system that could benefit from support. Furthermore, Free T4 is an inactive hormone, and your liver must convert it to Free T3 to activate it. Testing Free T3 provides the most accurate picture of the energy available for your reproductive system.

The optimal range for Free T3 is 5–7 pmol/L. If your result is at the low end of the standard range (near 3 pmol/L), you may still experience fatigue, hair loss, and fertility challenges despite being told your labs are "normal."

Several factors can impair the conversion of T4 to T3 in the liver. These are aging, inflammation, stress, severe illness or injury, calorie restriction, fasting, poor gut health, an imbalance of gut bacteria, and nutritional deficiencies such as zinc, selenium, vitamin D and iron. If you have concerns that you may have any of these issues, I recommend you work with a qualified practitioner to get a full thyroid panel done which will test TSH, Free T4, Free T3, Reverse T3 and thyroid antibodies.

When the Issue Isn’t the Thyroid (pituitary or adrenal causes)

"Sometimes the thyroid is healthy, it’s the pituitary that is not sending a strong enough signal"

Secondary ( or central) hypothyroidism occurs when the pituitary doesn’t send enough TSH. The thyroid is functioning fine, but the pituitary gland is failing to send a strong enough TSH message to the thyroid. This can result in lower than normal TSH levels and as a result, low T4 and T3 levels as well. This was the case with me as my TSH was often really low but I didn’t have any other symptoms or results to suggest it was hyperthyroidism (an overactive thyroid). Chronic stress and HPA (hypothalamic, pituitary, adrenal) axis dysfunction can often be the root cause of this dysfunctional TSH signalling .

Supporting the HPA axis through targeted nutrition and lifestyle practices can help preserve healthy central communication.

Nutrients that support central pituitary signalling and the secretion of TSH from the pituitary are zinc, vitamin A, magnesium, and vitamin B6.  Also, adaptogens such as Ashwagandha and Rhodiola help to support the HPA axis when under stress and can prevent high cortisol spikes from blunting central TSH signals.

The herb vitex (chaste tree) may work directly on the pituitary receptor pathways to support more balanced TSH signalling.

It is also important to protect your circadian rhythm. The pituitary gland naturally secretes TSH in nocturnal pulses (peaking overnight between 2:00 AM and 4:00 AM). Preserving this natural rhythm through a consistent sleep-wake timing and morning sunlight exposure provides the neurological cues your pituitary needs to thrive.

What to Do Next

If any of these signs resonate with you, a comprehensive thyroid panel is essential. It can reveal hidden issues that standard testing misses.

I specialise in helping women uncover these subtle patterns and create personalised plans to support thyroid health, hormonal balance, and reproductive wellbeing.

You’re welcome to book a FREE 15‑minute consultation, either at my clinic in Tauranga (Gate Pa Health Shop) or online via Zoom  to explore your next steps.