In this article

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When diagnosing someone with hypothyroidism, doctors and healthcare professionals generally look at clinical signs and changes in thyroid hormone production on a blood test. Common symptoms of hypothyroidism -- an underactive thyroid -- are assessed, such as intolerance to cold temperatures, dry skin and hair, menstrual issues, fatigue, unwanted weight gain, and depression. In addition, doctors include a complete blood test panel measuring thyroid levels –  including thyroid hormones T4 and T3 – to evaluate whether you have a thyroid condition.

Some experts believe that measuring a woman’s basal body temperature over time is a valuable way to monitor thyroid function, as unusually high or low temperatures may indicate problems with thyroid function. In this article, we’ll look at the link between a low basal body temperature and hypothyroidism. 

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Basal body temperature is the lowest temperature a woman’s body reaches in a 24-hour period, usually when sleeping or first thing in the morning while still at rest. The basal body temperature indicates how well your body’s metabolism is running.  

This measurement is done first thing in the morning, before moving or talking, as any activity can affect your results.

The basal body temperature thermometer differs from a standard thermometer because it shows two decimal places. A regular thermometer only shows one decimal place, for example, 98.1 degrees Fahrenheit versus 98.15 degrees Fahrenheit.

A temperature reading of 96 to 98 degrees Fahrenheit (35.5 to 36.6 Celsius) is typical before ovulation in women. After ovulation, women’s basal temperature can rise to 97 to 99 degrees Fahrenheit (36.1 to 37.2 Celsius). (Note: There is no average basal body temperature for men, as it’s a term used to describe the temperature of a woman’s body at rest.)

Basal body temperature monitoring is most commonly used to assess a woman’s menstrual cycles, including ovulation, luteal phase, and menstruation. Monitoring basal body temperature is often used to help determine fertility, pinpoint ovulation, and target optimal conception times to achieve pregnancy. 

If your basal body temperature is consistently lower than expected, it could also be a sign of hypometabolism – an underactive metabolism most commonly caused by hypothyroidism. 

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Thyroxine, also known as T4, is the storage form of thyroid hormone before it is converted into active T3. The level of T4 in your blood can indicate whether you have thyroid disease. T4 and body temperature can also help assess whether someone is in a hyperthyroid or hypothyroid state.

Because T4 helps to dilate blood vessels, it affects the body’s basal temperature. When too much T4 thyroxine is present in the bloodstream (hyperthyroidism), basal body temperature typically increases. A clinical sign of this is when a person feels too hot, even in a colder environment.

When T4 levels are too low (hypothyroidism), basal body temperature decreases, leading to feeling cold despite a warmer environment.

Most doctors don’t rely solely on basal body temperature measurement to diagnose or treat hypothyroidism. Still, some physicians, including Dr. Denis Wilson MD, author of Evidence-Based Approach to Restoring Thyroid Health, believe that measuring basal body temperature to assess thyroid health is potentially more reliable than blood tests alone. While Dr. Wilson’s position is controversial, some physicians include basal body temperature tracking along with the standard thyroid blood tests as part of their evaluation and monitoring. 

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To measure your basal body temperature, use a specific thermometer measuring up to 2 decimal places. You can purchase one of these thermometers at any pharmacy or drugstore or even order one online. Here are five things to keep in mind to effectively monitor your basal temperature: 

1) Take your temperature as soon as you wake up while you’re still in bed

2) Keep your thermometer at your bedside table and at arm’s reach to limit the amount of physical movement (which can alter the temperature reading)

3) Ideally, take your temperature at the same time each morning

4) Factors that will influence your body temperature include: alcohol the night before, stress, traveling between different time zones, and having a cold or flu

5) Track your temperature in your notes and let your doctor know if you notice a trend, such as consistent low temperatures or consistent high temperatures

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So what can be done if you have low basal body temperature? Treatment for this condition depends on the underlying cause. When it’s due to hypothyroidism, your thyroid hormone replacement medication should help regulate your hormones and improve your metabolic rate,  naturally raising the basal temperature. 

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If your basal body temperature is constantly measuring below the norm, it could indicate that you’re not producing enough body heat or energy needed for your cells. While measuring basal body temperature is not the gold standard for assessing the presence of thyroid dysfunction, it can be helpful when used in conjunction with a complete panel testing your thyroid hormone levels (such as Paloma Health’s complete thyroid blood test kit) and tracking any physical symptoms. These tools offer a comprehensive way to evaluate and support your thyroid health and healing.

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What is basal body temperature?

Basal body temperature is the lowest temperature your body reaches in a 24-hour period, usually measured first thing in the morning before you get out of bed. It reflects how well your metabolism is running and is commonly used to track ovulation and menstrual cycles.

How is basal body temperature connected to your thyroid?

Thyroid hormone, specifically T4, helps dilate blood vessels and affects your body's basal temperature. When T4 levels are too low, as in hypothyroidism, basal body temperature tends to decrease, which can cause you to feel cold even in a warm environment. When T4 is too high, as in hyperthyroidism, basal temperature tends to rise.

How do you measure basal body temperature correctly?

You'll need a basal body temperature thermometer that shows two decimal places. Take your temperature as soon as you wake up, before moving or talking, ideally at the same time each morning, and keep the thermometer within arm's reach to avoid extra movement. Alcohol, stress, travel, and illness can all affect your reading.

Can treating hypothyroidism raise a low basal body temperature?

Yes. When a low basal body temperature is caused by hypothyroidism, thyroid hormone replacement medication can help regulate your metabolism, which can naturally raise your basal temperature over time.

Is basal body temperature a reliable way to diagnose hypothyroidism?

Not on its own. Most doctors don't rely solely on basal body temperature to diagnose or treat hypothyroidism, but some use it alongside standard thyroid blood tests, such as a complete thyroid panel, as an additional way to monitor thyroid health.