Key takeaways
- Untreated hypothyroidism can make a TDEE estimate too high; treatment usually brings needs back to normal.
- Untreated hyperthyroidism raises energy use; after treatment, weight often returns, so recalculate.
- PCOS doesn’t appear to lower resting metabolism much, if at all; no single diet is better than others for it.
- Never take extra thyroid hormone or change a prescribed dose to lose weight.
On this page
- How the thyroid affects your TDEE
- Hypothyroidism: an underactive thyroid
- Hyperthyroidism: an overactive thyroid
- PCOS and metabolism
- Why weight can still feel harder with PCOS
- What the 2023 PCOS guideline recommends
- Using a TDEE calculator with a hormonal condition
- Measure your own TDEE
- Other hormonal and medical factors
- When to see a doctor
- Questions about TDEE with PCOS and thyroid issues
How the thyroid affects your TDEE
Thyroid hormone helps set the pace at which your body uses energy. In the American Thyroid Association’s words, high thyroid hormone levels raise basal metabolic rate and low levels decrease it. Resting metabolism is the largest part of your TDEE, so a thyroid that is working too hard or not hard enough can shift your daily needs. But it is only one of many factors, and the effect is usually smaller than people expect.
Hypothyroidism: an underactive thyroid
When the thyroid makes too little hormone, resting metabolism falls and people often gain some weight. According to the American Thyroid Association, most of that weight is salt and water, and for most patients about 5 to 10 lb (2 to 4.5 kg) of weight gain may be related to the thyroid. Larger gains usually have other causes as well.
Treatment with thyroid hormone, usually levothyroxine, returns body weight to roughly where it was before the thyroid became underactive. If other symptoms improve with treatment but weight doesn’t, the gain probably wasn’t caused by the thyroid alone. What this means for planning:
- Before treatment, a TDEE calculator may overestimate your needs. Focus on getting diagnosed and treated first rather than on a calorie target.
- Once your dose is stable and your blood tests are in range, standard formulas apply. Use your current weight and an honest activity level.
- Check your own data after three to four weeks, as anyone should.
Never take extra thyroid hormone, or change your dose, to lose weight. Too much thyroid hormone can cause a fast or irregular heartbeat and bone loss, and any change in dose should be made by your doctor based on blood tests.
Hyperthyroidism: an overactive thyroid
With too much thyroid hormone, resting metabolism rises and many people lose weight without trying. Not everyone does: hyperthyroidism also raises appetite, so some people gain weight instead. After treatment, the weight lost because of excess thyroid hormone usually comes back as hormone levels return to normal. Some people also keep eating the larger amounts they got used to while their thyroid was overactive.
This is the point where a TDEE calculator is most useful. Once your levels are back in range, recalculate your TDEE at your current weight and compare it with what you are actually eating. If your appetite has stayed high, regular meals with plenty of protein and fibre can help you settle back to your new needs. Don’t try to diet while hyperthyroidism is untreated; it needs medical treatment first.
PCOS and metabolism
Polycystic ovary syndrome is one of the most common hormonal conditions in women of reproductive age, affecting roughly one in ten. It is linked to irregular periods, higher levels of androgens (hormones such as testosterone), insulin resistance and a higher risk of weight gain. Many women with PCOS are told their metabolism is slow. The research is more reassuring than that.
- A 2009 study found lower basal metabolic rate in women with PCOS who had high androgen levels, and linked it to insulin resistance.
- A 2016 study comparing women with and without PCOS found that resting energy expenditure was no lower in the women with PCOS.
- A 2025 systematic review and meta-analysis of 13 studies with 918 participants, so far published as a preprint, estimated a difference of about 30 kcal a day, with almost no difference once body size was taken into account. Its authors concluded that a slower metabolism is unlikely to explain weight challenges in PCOS.
In other words, a standard TDEE formula is usually a reasonable starting point if you have PCOS. There is no good reason to cut your estimate by hundreds of calories “because of PCOS”.
Why weight can still feel harder with PCOS
If metabolism isn’t the main issue, why do so many women with PCOS find weight management hard? Several things play a part:
- Insulin resistance is common in PCOS and can affect appetite and where fat is stored.
- Irregular cycles make the scale harder to read, because water retention arrives at unpredictable times.
- Disordered eating is more common. The 2023 international PCOS guideline says eating disorders and disordered eating should be considered in PCOS regardless of weight.
- Weight stigma. The same guideline notes that many women with PCOS experience weight stigma in healthcare, which can make seeking help harder.
What the 2023 PCOS guideline recommends
The 2023 International Evidence-based Guideline for the Assessment and Management of PCOS, developed by experts and people with PCOS from around the world, makes several practical points:
- No single diet is best. It states there is no evidence to support any one type of diet composition over another, and recommends sustainable healthy eating tailored to your preferences and goals.
- If weight loss is a goal, a tailored energy deficit can be prescribed, taking into account your energy needs, body weight and activity.
- Move regularly. For adults, at least 150 to 300 minutes a week of moderate activity, or 75 to 150 minutes of vigorous activity, plus muscle-strengthening on two non-consecutive days. For modest weight loss and to prevent regain, at least 250 minutes a week of moderate activity.
Lifestyle changes help PCOS symptoms and long-term health even when weight changes little. Your doctor may also discuss medicines alongside them.
Using a TDEE calculator with a hormonal condition
| Situation | What to expect | What to do |
|---|---|---|
| Untreated underactive thyroid | Estimate may be too high | Get treated first; measure later |
| Treated thyroid, stable blood tests | Estimate works as normal | Use the calculator, then check your data |
| Untreated overactive thyroid | Needs are higher than the estimate | Don’t diet; see your doctor |
| After treatment for an overactive thyroid | Weight often returns | Recalculate at your new weight |
| PCOS | Estimate is usually reasonable | Use it as a start; measure over 4 to 6 weeks |
Measure your own TDEE
Whatever your condition, your own data is the most reliable guide. Eat a steady, logged amount for several weeks, weigh yourself most mornings and compare weekly averages. With irregular periods, use a longer window of four to six weeks so that water swings even out. Then:
Measured TDEE = average daily intake − (change in weekly average weight in kg × 7,700 ÷ days)
For example, a 32-year-old woman with PCOS, 165 cm (5′5″) and 88 kg (194 lb), lightly active, has an estimated TDEE of about 2,187 kcal. She logs an average of 1,800 kcal, and her weekly average falls by 0.9 kg over three weeks. Her measured TDEE is 1,800 + 0.9 × 7,700 ÷ 21 = about 2,130 kcal, within about 60 kcal of the estimate. The adaptive TDEE calculator does this for you from your log.
Other hormonal and medical factors
Thyroid conditions and PCOS are the most common questions, but other factors can affect weight and appetite: menopause (see TDEE for women over 40), rarer hormonal conditions such as Cushing’s syndrome, and some medicines, including steroid tablets, some antidepressants and antipsychotics, and some diabetes treatments. If you think a medicine is affecting your weight, talk to your doctor. Don’t stop or change it on your own.
When to see a doctor
See your doctor if you notice symptoms that could point to a thyroid problem, such as unexplained weight change, tiredness, feeling unusually cold or hot, a racing heart, constipation or dry skin, or symptoms that could point to PCOS, such as irregular or absent periods, excess hair growth, acne or difficulty getting pregnant. Also see them if you have been eating carefully below your estimated TDEE for two months without any change. A simple blood test can rule a lot out, and getting a condition treated will do more for your energy needs than any formula.
Questions about TDEE with PCOS and thyroid issues
Does hypothyroidism lower your TDEE?
Yes, while it is untreated, because low thyroid hormone lowers resting metabolism. The related weight gain is usually modest and mostly salt and water. With treatment, energy needs usually return to normal.
How many calories should I eat with hypothyroidism?
Once your treatment is stable, start from a standard TDEE estimate at your current weight and activity, then adjust from three to four weeks of your own data. Your doctor or a registered dietitian can help you set a target.
Will levothyroxine help me lose weight?
It treats the thyroid problem and usually reverses the weight gained because of it, which is often a few kilos. It isn’t a weight-loss medicine, and the dose should never be changed for weight loss.
Does PCOS slow your metabolism?
Probably not by much. Studies disagree, but the largest pooled analysis so far found a difference of only about 30 kcal a day, and almost none once body size was taken into account.
What is the best diet for PCOS?
The 2023 international guideline found no evidence that any one diet composition is better than another. Choose a healthy way of eating you can keep up, and include regular activity.
Should I use a different TDEE formula if I have PCOS?
No. A standard formula is a reasonable start. Then measure your real TDEE from four to six weeks of food and weight data, especially if your periods are irregular.
How does hyperthyroidism affect calorie needs?
It raises them while it is untreated, so many people lose weight. After treatment, needs fall back to normal and the weight often returns, which is the time to recalculate.
Sources
- American Thyroid Association. Thyroid and weight. Patient brochure. www.thyroid.org/wp-content/uploads/patients/brochures/Thyroid_and_Weight.pdf
- Teede HJ, Tay CT, Laven J, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Hum Reprod. 2023;38(9):1655-1679. doi.org/10.1093/humrep/dead156
- Georgopoulos NA, Saltamavros AD, Vervita V, et al. Basal metabolic rate is decreased in women with polycystic ovary syndrome and biochemical hyperandrogenemia and is associated with insulin resistance. Fertil Steril. 2009;92(1):250-255. pubmed.ncbi.nlm.nih.gov/18678372/
- Larsson I, Hulthén L, Landén M, et al. Dietary intake, resting energy expenditure, and eating behavior in women with and without polycystic ovary syndrome. Clin Nutr. 2016;35(1):213-218. pubmed.ncbi.nlm.nih.gov/25743212/
- Resting energy expenditure of women with and without polycystic ovary syndrome: a systematic review and meta-analysis. medRxiv preprint, not yet peer reviewed. 2025. www.medrxiv.org/content/10.64898/2025.12.03.25341536.full.pdf
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. doi.org/10.1093/ajcn/51.2.241