7 Medical Reasons You're Struggling to Lose Weight

Solcara Health Blog

Ashley Harwyn, PA-C, ABAAHP | Published August 4, 2025 | Last updated October 6, 2026 | 19 min read

Thyroid, medications, sleep, PCOS, menopause, insulin resistance, and metabolic adaptation after past weight loss: what each one does, how big the measured effect is, and how it is tested.

Smiling woman with shoulder-length wavy hair, wearing a dark pullover and black leggings, standing with her arms crossed in a bright room with tall windows and a large potted plant — When the scale stalls, the useful next step is finding out why, one checkable cause at a time.

If you live in Charleston or Mount Pleasant and cannot lose weight despite real effort, a medical reason may be worth checking before anyone blames willpower. Seven worth ruling out are an underactive thyroid, a medication with weight gain as a side effect, short sleep, polycystic ovary syndrome (PCOS), the menopause transition, insulin resistance, and the way the body pushes back after earlier diets.[1][4][5][7][8][9][10] Each section below names the test or question that helps sort it out, and our medical weight loss program page describes an assessment that starts with your history, then labs chosen from it, then a body composition baseline.

This guide takes the seven one at a time. For each, you will find what the cited research shows and how large the measured effect was. The effects differ a lot in size; the thyroid's share, for example, is usually about 5 to 10 pounds.[2]

Key takeaways

An underactive thyroid usually accounts for about 5 to 10 pounds, mostly salt and water, and a TSH blood test is the first check.[2][3]
In a national CDC survey, 20.3% of US adults were taking at least one prescription medicine linked to weight gain in 2017 to 2018, up from 13.2% in 1999 to 2000.[4]
In a small controlled study, when dieters had a 5.5 hour sleep window instead of 8.5 hours, the fraction of weight they lost as fat fell by 55% and they lost more lean mass.[5]
PCOS affects roughly 10% to 13% of reproductive-aged women and is linked with greater weight gain over time, and the 2023 guideline found no evidence that any one diet composition beats another for it.[7]
Fat mass rises faster during the menopause transition (1.7% per year versus 1.0% before it), while lean mass starts to fall.[8]
In a six-year follow-up of 14 people after a 30-week weight loss competition, resting metabolism stayed lower than predicted from their body composition and age.[10]

Section 01 / Thyroid

Can an underactive thyroid stop you from losing weight?

It can contribute, within limits. When the thyroid makes too little hormone (hypothyroidism), many body functions slow down, and weight gain is one of the listed symptoms, along with fatigue, trouble tolerating cold, dry skin, joint and muscle pain, and heavy or irregular periods.[1] Nearly 5 out of 100 Americans ages 12 and older have hypothyroidism, and women and people over 60 are more likely to develop it.[1]

The American Thyroid Association puts the thyroid's share at about 5 to 10 pounds, depending on how severe the hypothyroidism is, and says most of that gain is retained salt and water rather than fat.[2] The ATA says treatment returns body weight to what it was before the hypothyroidism started.[2] The ATA adds that if the other hypothyroid symptoms improve with treatment, it is unlikely that the weight gain was only due to the thyroid.[2]

NIDDK makes the same point from the other side: fatigue and weight gain are so common that, on their own, they “do not necessarily mean you have a thyroid problem.”[1]

How it is checked. A TSH (thyroid stimulating hormone) blood test is the recommended first step when symptoms raise the question. A free T4 level separates clear hypothyroidism from a milder, subclinical form, and if TSH is in the normal range, the review says other causes for the symptoms should be sought.[3] The American Academy of Family Physicians review notes that most people with subclinical hypothyroidism do not benefit from treatment unless TSH is above 10 mIU per L or the thyroid peroxidase (TPO) antibody is elevated.[3] Extra thyroid hormone is not a weight loss shortcut either. The ATA notes that any weight lost that way is usually regained once the excess hormone stops.[2] If your numbers are off, our thyroid management page covers thyroid testing and treatment in Mount Pleasant and downtown Charleston.

Section 02 / Medications

Could one of your medications be causing weight gain?

Some widely used prescriptions are linked to weight gain. The Endocrine Society's obesity guideline, as used in a CDC analysis, names certain medicines in six classes as weight-promoting: some beta-blockers, some diabetes medicines, some anticonvulsants, some antidepressants, some corticosteroids, and some antipsychotics.[4] Among adults taking one of those anticonvulsants, nerve pain (neuralgia or neuritis) was the most common reason for use.[4] In that analysis, the other medicines in these classes were classed as not weight-promoting.[4]

How common is it? A CDC analysis of national survey data found that 20.3% of US adults took at least one of these weight-promoting medicines in 2017 to 2018, up from 13.2% in 1999 to 2000. Beta-blockers were the most common (9.8% of adults), followed by diabetes medicines (5.7%) and anticonvulsants (4.5%).[4] Published estimates of the weight gain tied to individual drugs ranged from 0.3 to 15.3 kg, roughly two-thirds of a pound to 34 pounds, so the effect varies a great deal from one medicine to the next.[4] Antihistamines were also identified as weight-promoting, but they were not included in that analysis.[4]

Whether to switch is a prescribing decision. The CDC authors put it plainly: the decision to prescribe an alternative without the weight effect, if one exists, is guided by weighing the risks and benefits of the available treatments.[4] Bring that question, and the list of everything you take, to the prescriber.

How it is checked. Bring every prescription, over-the-counter medicine, and supplement to your visit, with the date you started each one. Then line those dates up against your weight history, and raise any gain that began after a new prescription with the prescriber. For readers who are also using hormones, our post on whether hormone replacement therapy helps with weight loss covers that separate question.

Section 03 / Sleep

How much does poor sleep get in the way of weight loss?

One small, controlled study published in the Annals of Internal Medicine looked at this directly. Ten adults with overweight followed the same moderate calorie cut for 14 days twice, once with an 8.5 hour sleep window and once with 5.5 hours, in a randomized two-period crossover design.[5]

The scale moved in both rounds. What changed was where the weight came from. With short sleep, the fraction of weight lost as fat dropped by 55% (1.4 kg of fat with full sleep versus 0.6 kg with short sleep), and the loss of lean, fat-free mass rose by 60% (1.5 kg versus 2.4 kg). Participants also reported more hunger when sleep was cut.[5] The authors concluded that too little sleep “may compromise the efficacy” of a standard diet.[5]

Population studies point the same way, with an important caveat. A meta-analysis pooling data from more than 600,000 adults found short sleepers had higher odds of obesity (a pooled odds ratio of 1.55), and each extra hour of sleep was linked with a BMI about 0.35 units lower.[6] The authors were careful to say that causal inference from these studies is difficult, partly because important confounders were not controlled.[6]

Sleep apnea deserves a separate mention. NIDDK lists it among the conditions that can raise the chance of developing insulin resistance, and the international PCOS guideline asks clinicians to assess women with PCOS for its symptoms, such as snoring together with waking unrefreshed, daytime sleepiness, or fatigue.[7][9] If those sound familiar, raise them at your visit.

How it is checked. A sleep log (bedtime, wake time, night waking, and how rested you feel) gives your clinician something concrete to review. If you snore and wake unrefreshed, ask whether a sleep apnea evaluation makes sense for you.[7]

Section 04 / PCOS

Does PCOS make it harder to lose weight?

It can. Polycystic ovary syndrome is common in reproductive-aged women, with a prevalence between 10% and 13%, according to the 2023 international evidence-based guideline.[7] The guideline states that many women with PCOS “will have underlying mechanisms that drive greater longitudinal weight gain and higher BMI,” even though the exact mechanisms are not fully understood.[7] The guideline calls insulin resistance a pathophysiological factor in PCOS.[7]

The guideline diagnoses PCOS when two of three features are present: hyperandrogenism, meaning excess androgens (the hormones often called male hormones), shown by clinical signs or blood tests; ovulatory dysfunction, such as irregular cycles; and polycystic ovaries on ultrasound.[7] Clinical signs of excess androgens it lists include acne, female pattern hair loss, and hirsutism (extra facial and body hair).[7] The guideline notes that when someone has both irregular cycles and signs of excess androgens, an ovarian ultrasound “is not necessary for PCOS diagnosis.”[7] When it is needed, a blood test called AMH can be used instead of the ultrasound in adults, but the guideline says AMH should not be used as a single test for the diagnosis.[7]

Two more findings from the guideline are practical. First, it found no evidence that any one type of diet composition works better than another for weight, metabolic, hormonal, or reproductive outcomes in PCOS.[7] Second, it states that clinically available insulin blood tests are of limited clinical relevance and should not be used in routine care.[7] It recommends the 75 g oral glucose tolerance test as the most accurate test of blood sugar status in PCOS, regardless of BMI.[7]

The guideline also states that many women with PCOS experience weight stigma in health care, and asks clinicians to be aware of it when discussing lifestyle.[7] What it does recommend for all women with PCOS is lifestyle intervention (exercise alone, or diet combined with exercise and behavioral strategies) to improve metabolic health, with metformin considered for adults with PCOS and a BMI of 25 or higher.[7]

How it is checked. The guideline points to a history and physical exam for signs of excess androgens, androgen blood levels when signs are minimal, a glucose tolerance test, and, when needed, AMH or an ultrasound.[7] Hormone testing is part of our gynecology and fertility care, and our menstrual mapping page explains what a cycle map can and cannot show, including for PCOS.

Section 05 / Menopause transition

Why do perimenopause and menopause change your weight?

Long-term data on this come from the Study of Women's Health Across the Nation (SWAN), which followed 1,246 Black, Chinese, Japanese, and White women with repeated DXA body composition scans for years around their final period.[8]

Before the menopause transition, fat mass rose about 1% per year. During the transition, that pace accelerated to 1.7% per year. At the same time, lean mass, which had been inching up by about 0.2% per year, began falling by about 0.2% per year.[8] The gains and losses continued until about two years after the final menstrual period, then leveled off.[8]

Here is the surprising part. Weight on the scale showed no discernable change in its rate of gain at the start of the transition, because the sum of fat and lean mass rose at a similar pace before and during it.[8] In this study, the shift in body composition at the start of the transition did not show up as a change in the rate of weight gain.[8]

For women who also have hot flashes, poor sleep, or other menopause symptoms, our post on perimenopause and functional medicine in Mount Pleasant covers the broader picture, and our bioidentical hormone therapy page covers hormone therapy itself.

How it is checked. Cycle history and symptoms come first. A body composition analysis estimates fat and lean mass; our page on it explains that the scan is better at showing change over time than at giving one exact number.

Not sure which of these is slowing you down? Call 843-981-0870. Ask about reviewing your medicines, sleep, cycles, and labs at your visit.

Section 06 / Blood sugar

Is insulin resistance or prediabetes part of the picture?

Insulin resistance means your body does not respond to insulin the way it should. NIDDK notes that it can lead to higher blood sugar and to weight gain, and that if blood sugar keeps going up, type 2 diabetes can follow.[9] About 97.6 million American adults had prediabetes in 2021, according to NIDDK.[9] The catch is that insulin resistance and prediabetes usually cause no symptoms at all.[9]

NIDDK's list of risk factors includes overweight or a large waist, age 35 or older, a family history of diabetes, physical inactivity, smoking, a history of gestational diabetes, and conditions such as PCOS and sleep apnea. Certain medicines, including glucocorticoids and some antipsychotics, raise the risk too.[9] Several of those, including PCOS, sleep apnea, and certain medicines, also appear elsewhere on this list.

Testing is worth being precise about. NIDDK states that health care professionals may not test for insulin resistance directly, because the test for it “is primarily used only for research studies.”[9] What NIDDK describes instead is blood sugar testing, with these cutoffs for prediabetes: an A1C of 5.7% to 6.4%, a fasting plasma glucose of 100 to 125 mg/dL, or a two-hour oral glucose tolerance test result of 140 to 199 mg/dL.[9]

In the Diabetes Prevention Program, an NIH-funded study, people at high risk of type 2 diabetes who lost 5% to 7% of their starting weight lowered their chance of developing it.[9] For someone who weighs 200 pounds, 5% to 7% is 10 to 14 pounds.

How it is checked. Blood sugar testing (A1C and glucose) is part of our annual full panel of labs, and the PCOS guideline favors a glucose tolerance test in women with PCOS.[7][9]

Checks named in this guide

A TSH blood test, every medication with its start date, your sleep, your cycle history, an A1C or fasting glucose, and your history of past weight loss and regain. Ordinary checks, read together.

What the cited sources do not support

Extra thyroid hormone as a weight loss tool (the weight lost is usually regained once it stops), routine insulin blood tests in PCOS (limited clinical relevance), and any one diet composition as the best one for PCOS.

Section 07 / Metabolic adaptation

Is your body fighting back after past diets?

If you have lost a lot of weight before and watched it return, this one is worth understanding. Researchers call it metabolic adaptation: the part of the change in resting metabolism that remains after adjusting for changes in body composition and age.[10]

One long-term example comes from a follow-up of contestants from the televised weight loss competition The Biggest Loser. Fourteen of the original 16 contestants were measured again six years later. They had lost an average of 58.3 kg (about 128 pounds) by the end of the competition and had regained an average of 41.0 kg (about 90 pounds) six years on.[10] Their resting metabolic rate had fallen by about 610 calories a day at the end of the show. Six years later, it was still about 704 calories a day below where it started, and metabolic adaptation was still about 499 calories a day.[10]

Two cautions keep this in proportion. The group was small (14 people), and the weight loss came from a 30-week televised competition.[10] Still, the authors concluded that metabolic adaptation “persists over time” and is likely a proportional, but incomplete, response to ongoing efforts to reduce body weight.[10] In the sleep study above, hunger also rose when sleep was short.[5]

Large, fast loss like the kind on that show is the pattern studied here. Our post on why nutritional therapy works better than diets goes deeper on keeping weight off, and what to expect from a personalized weight loss plan walks through how a plan is structured.

Here are all seven side by side, with what to notice and what actually checks each one.

Source: Hales CM et al, Obesity, 2022 (CDC National Center for Health Statistics analysis of NHANES). Classes as defined by the Endocrine Society. Only certain drugs within each class carry the weight effect.

Reason	Clues worth noticing	What checks it	What the cited source measured
Underactive thyroid	Fatigue, trouble tolerating cold, dry skin, heavy or irregular periods[1]	TSH, with free T4 to separate clinical from subclinical[3]	Thyroid-related gain of about 5 to 10 pounds[2]
Medication side effect	Taking a medicine from one of the six flagged classes[4]	A full medication review with start dates	Gain of 0.3 to 15.3 kg, depending on the drug[4]
Short or poor sleep	Short nights; snoring with waking unrefreshed[7]	Sleep history; apnea evaluation if symptoms fit[7]	55% smaller fraction of weight lost as fat in a 10-person trial[5]
PCOS	Irregular cycles, acne, hirsutism[7]	History and exam, androgens, glucose tolerance test, AMH or ultrasound if needed[7]	Prevalence of 10% to 13%[7]
Menopause transition	Years around the final period[8]	Cycle history, body composition	Fat mass gain of 1.7% per year during the transition[8]
Insulin resistance or prediabetes	Usually no symptoms; family history, large waist[9]	A1C, fasting glucose, glucose tolerance test[9]	97.6 million US adults with prediabetes in 2021[9]
Body defending lost weight	A large past weight loss, then regain[10]	Your weight and diet history	Metabolic adaptation of about 499 calories a day at six years, in 14 people[10]

Before your weight loss visit

Which of these apply to you?

Tap each statement that fits. None of them diagnoses anything. Each one points to a question worth raising at your visit.

Read down the third column and you will notice that most of these checks are ordinary: a blood draw, a medication list, a sleep log, a history.

Medical weight loss

History first, then labs, then a body composition baseline. Read Medical weight loss.

Thyroid management

TSH, free T4, and thyroid treatment when testing shows a problem. Read Thyroid management.

Annual full panel labs

A1C, glucose, and the rest of a yearly baseline panel. Read Annual full panel labs.

What to expect from a personalized weight loss plan

The step-by-step process once the causes are on the table. Read What to expect from a personalized weight loss plan.

Ask Ashley

Ashley Harwyn, PA-C PA-C, ABAAHP · Founder, Solcara Health Online

Tap a question to ask

Why can't I lose weight even though I eat well and exercise? Effort is only one input. Several medical factors can slow progress even when habits are good: an underactive thyroid, a medicine with weight gain as a side effect, short sleep, PCOS, the menopause transition, insulin resistance, and the lower resting metabolism that can follow earlier weight loss. Most of these can be looked at with a careful history, a medication review, and routine blood work such as TSH and A1C. The size of each effect differs, from about 5 to 10 pounds for an underactive thyroid to a 55% smaller fraction of fat lost with short sleep in one small trial. Ask which of them fit your history.

Can a normal thyroid test still be the reason I'm gaining weight? TSH is the recommended first test when symptoms raise the question, and an American Family Physician review says that when TSH is in the normal range, other causes for the symptoms should be sought. Free T4 separates clinical from subclinical hypothyroidism. Even true hypothyroidism usually accounts for about 5 to 10 pounds, mostly salt and water, according to the American Thyroid Association. Most people with subclinical results do not benefit from treatment unless TSH is above 10 mIU per L or the TPO antibody is elevated. A normal result is a reason to look at the other causes on this list too.

Which medications commonly cause weight gain? The Endocrine Society's obesity guideline flags certain medicines within six classes: some beta-blockers, some diabetes medicines, some anticonvulsants, some antidepressants, corticosteroids, and some antipsychotics. Antihistamines were also identified as weight-promoting but were left out of the analysis. A CDC analysis found that 20.3% of US adults took at least one of these in 2017 to 2018, and it classed the other medicines in these classes as not weight-promoting. The authors note that the decision to prescribe an alternative, if one exists, is guided by weighing the risks and benefits of the available treatments. Raise that question with your prescriber.

Does lack of sleep really affect weight loss? It appears to. In a controlled crossover study of 10 adults on the same calorie cut, the fraction of weight lost as fat fell by 55% and the loss of lean mass rose by 60% when they had a 5.5 hour sleep window instead of 8.5 hours, and hunger increased. Large population studies also link short sleep with obesity, though their authors say causal inference is difficult. Snoring with waking unrefreshed is worth mentioning, since NIDDK lists sleep apnea among the conditions that can raise the chance of insulin resistance.

Is there a test for insulin resistance? Not a routine one. NIDDK states that the test for insulin resistance is primarily used in research, and the international PCOS guideline says clinically available insulin tests are of limited clinical relevance. What is measured instead is blood sugar status: an A1C of 5.7% to 6.4%, a fasting glucose of 100 to 125 mg/dL, or a two-hour glucose tolerance result of 140 to 199 mg/dL indicates prediabetes. In the Diabetes Prevention Program, people at high risk who lost 5% to 7% of their starting weight lowered their chance of developing type 2 diabetes.

Where can I get checked for medical causes of weight gain in Charleston or Mount Pleasant? We see patients at our Mount Pleasant office on Bramson Court and at The Longevity Club on Rutledge Avenue in downtown Charleston. Our medical weight loss page describes the order of an assessment: your history first, then labs chosen from it, then a body composition baseline. Topics worth raising include your weight history, every medication you take, your sleep, and, for women, your cycle history. You can call 843-981-0870 or book online. Bring any recent lab results and a list of your medicines with start dates.

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About the author

Ashley Harwyn, PA-C, founder of Solcara Health in Charleston, South Carolina.

Ashley Harwyn, PA-C, ABAAHP

Founder, Solcara Health · Diplomate, American Board of Anti-Aging Health Practitioners (ABAAHP)

Ashley has practiced medicine for more than fifteen years. Her path into it began at home, with a family member born with a congenital heart defect and a childhood spent watching how much the quality of a clinician's attention could change the shape of a life. She has since practiced across a range of settings, including medical mission work in Uganda, before founding Solcara to build the kind of practice she kept wishing existed, one that treats patients as whole people and tells them the truth about what the evidence does and does not support.

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Medical Disclaimer: This page is for educational and informational purposes only and does not constitute medical advice, and reading it does not create a patient-provider relationship. It describes general medical information and categories of care, not a recommendation that any individual start, stop, or change any test, treatment, or medication. Any diagnosis or treatment decision requires evaluation by a qualified, licensed clinician who knows your personal and family history. Consult a qualified healthcare provider before making medical decisions. Information reflects sources available as of October 5, 2026.

Ashley Harwyn, PA-C, ABAAHP, is a Diplomate of the American Board of Anti-Aging Health Practitioners (ABAAHP) through A4M and the founder of Solcara Health. Solcara serves South Carolina from Downtown Charleston (163 Rutledge Ave, Ste 202, The Longevity Club) and Mt. Pleasant (496 Bramson Ct, Ste 120).

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