3 Ways Your Metabolism Changes Throughout the 4 Phases of the Menstrual Cycle

The 4 phases of the menstrual cycle are marked by significant reproductive hormone fluctuations, which impact our metabolism! The key hormones at play are estrogen, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). As these hormones fluctuate and our metabolism shifts, we experience changes in energy levels, how well our body is able to process sugar, and even thyroid function. Let’s dive into the 3 main metabolic changes that occur throughout the menstrual cycle!

Overview of the 4 Phases of the Menstrual Cycle

This first day of the menstrual cycle is marked by the first day of bleeding, or menstruation. During the menstrual phase, estrogen, progesterone, FSH and LH are all collectively at their lowest point. Next, we move into the follicular phase, in which follicles develop within the ovaries thanks to increasing FSH levels. This is all in preparation for ovulation and potential egg fertilization. 

The increase in FSH stimulates estrogen to rise and LH to surge, which marks the ovulatory phase. While the ovulatory phase lasts about 3-4 days, ovulation occurs only when an egg is released. The surge in LH triggers progesterone production. Finally, these progesterone levels rise throughout the luteal phase, peaking at the midpoint of the luteal phase. Toward the end of the luteal phase, if fertilization does not occur, estrogen levels will rise. This blunts progesterone production to stimulate menstruation and the next menstrual cycle.

Change #1 – Alterations to Glucose Metabolism

Glucose (sugar) is the body’s main source of energy and is a primary component of many carbohydrate food sources. The way that we process glucose is impacted by hormone levels and fluctuations throughout the menstrual cycle. Glucose metabolism is the way that our body breaks down glucose to produce chemical energy, which happens in our mitochondria.

Estrogen levels increase toward the end of the follicular phase, as we approach ovulation. This increase in estrogen boosts glucose metabolism and insulin sensitivity. Insulin is the hormone that moves glucose from the bloodstream into the tissues, mainly our muscle and liver. Insulin sensitivity is how well insulin is able to control blood sugar levels, essentially how effectively we are able to metabolize glucose. 

That being said, insulin sensitivity and glucose metabolism improve when estrogen levels increase, such as during the late follicular phase. During the late luteal phase, as we approach menstruation, estrogen levels decline along with glucose metabolism. We also interestingly see a decrease in myo-inositol levels during the luteal phase, which contributes to a decrease in insulin sensitivity. 

Research shows more insulin resistance during the luteal phase compared to other times in the cycle. However, there is also evidence to suggest that the extent to which glucose metabolism decreases during the luteal phase depends on our progesterone levels. Progesterone is a counter-hormone to estrogen and is believed to also lower glucose metabolism.

This may be something to take into account when having your blood sugar and insulin levels tested or if utilizing a device like a continuous glucose monitor.

Change #2 – Drop in Thyroid Hormones

We know that thyroid dysfunction can have significant downstream effects on reproductive hormones and menstrual cycles. Both hypothyroidism, slow thyroid function, and hyperthyroidism, overactive thyroid function, are linked to hormone imbalances. Women with hypothyroidism are more likely to experience infrequent periods, elongated cycles, as well as excessive bleeding. Whereas those with hyperthyroidism are more likely to have lighter and shorter bleeds.

We know how strongly thyroid function influences hormone regulation downstream, but hormone levels also impact thyroid activity upstream. The estrogen and progesterone fluctuations that occur throughout the phases of the menstrual cycle lead to shifts in thyroid activity. One of the main roles of thyroid hormones is to regulate the speed of metabolism. So, any shifts in thyroid activity directly impact our metabolism.

In the follicular phase when progesterone is at its lowest, thyroid hormones are also at their low point. In addition to progesterone, thyroid hormones, both inactive T4 and active T3, have a positive correlation with estrogen levels throughout the cycle. Research has shown that women with higher total T4, total T3, and free T3 had higher levels of estrogen at various times throughout the cycle.

It’s not until later in the follicular phase when estrogen levels start to increase, at which point we may see an increase in thyroid hormone levels. This also comes with a major increase in thyroid stimulating hormone (TSH) levels right around the time of ovulation in response to increased estrogen levels, as long as there is no thyroid dysfunction at play. Keep this in mind if you are getting your thyroid blood labs tested around this time!

During the luteal phase, research suggests an increase in reverse T3, which is an inactivated form of thyroid hormone T3. Reverse T3 increases during times of stress and illness. During the late luteal phase, we experience an increase in inflammatory hormone-like compounds called prostaglandins, and a decrease in estrogen and progesterone, which have anti-inflammatory effects. 

Change #3 – Shift in Resting Metabolic Rate (RMR)

Resting metabolic rate (RMR) is the amount of total energy, or calories, that our body burns in one day while completely at rest (doing absolutely nothing). A decrease in RMR means our body uses less energy, or burns fewer calories. An increase in RMR means that our body requires more fuel to sustain essential body functions. RMR does not take into account our activity level or the energy used to digest the food we eat.

We experience our lowest RMR during early menstruation, with progesterone and estrogen at their lowest points. Toward the end of our period, as estrogen increases slightly, RMR also begins to increase. 

During the luteal phase, RMR increases significantly as progesterone levels increase to their highest point. Progesterone is known to have what’s called a thermogenic effect, meaning it causes the body to heat up. This internal heat helps ramp up metabolism and raises our RMR.

This aligns with what we know about thyroid hormones, and their role in regulating the speed of metabolism. We see thyroid hormone levels increase during the luteal phase, resulting in the speeding up of metabolism, and therefore an increase in RMR.

How to Learn More About Your Hormonal Shifts

As you can see, there is so much shifting throughout the hormonal cycle! Fluctuations in our reproductive hormones impact glucose metabolism, thyroid function, and overall speed of metabolism. If you are experiencing hormonal imbalances, these normal metabolic changes can be exacerbated and promote further imbalance. If you’re not sure where to start, be sure to track your menstrual cycle and consider advanced functional hormone testing for additional insight!

Written by Romana Brennan, MS, RD