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Is Phentermine Safe For Weight Loss With Hypothyroidism?

Phentermine is a weight loss medication. Learn if this drug has any contraindications with your thyroid condition.
Is Phentermine Safe For Weight Loss With Hypothyroidism?
Last updated:
7/19/2022
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Most of us know that healthy eating and exercise habits are the best tools for helping us keep our weight in check. However, some people cannot maintain a healthy weight, even when they do everything “right.” Thus, certain medications, like phentermine, may be helpful for people looking to shed unnecessary weight. And, unlike other weight loss pills, phentermine is proven effective for short-term weight loss, but it doesn’t come without side effects and risks. Here is what you need to know about taking phentermine with hypothyroidism.

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What is phentermine?

 

Phentermine is a medication that helps people lose weight. Your doctor needs to prescribe this medication for you. You may see phentermine sold under the brand names Adipex-P, Lomaira, or Suprenza. It is also under the brand name Qsymia, which is a combination of phentermine and other chemicals that promote weight loss.

 

This medication is among the class of stimulants. Medications in this class have similar chemical properties to more well-known stimulants like amphetamine. Thus, it is a controlled substance and requires careful monitoring from your healthcare provider while taking it.

 

Phentermine has been available for weight loss since 1959, when the FDA approved it for people seeking weight loss. However, it was only approved for use for three months in adults. Since then, this medication has been used in conjunction with other chemicals to help promote weight loss. However, the FDA removed many of these medications from the market as it caused heart problems and other health complications. 

 

You must be obese with a body mass index (BMI) of at least 30 or more to take this medication. However, it may also be prescribed if have a BMI of 27 or greater, and have at least one health condition related to obesity, such as type 2 diabetes or high blood pressure.  

 

How does phentermine help with weight loss?

 

Phentermine works primarily by suppressing your appetite. Indeed, it is considered an “anorectic,” which helps you limit the number of calories you eat in one day. As you may imagine, this can help people lose weight after being on this medication for a while. 

 

There is still some speculation about how anorectics like phentermine suppress your appetite. There is likely a relationship between this medication and dopamine, serotonin, and norepinephrine levels in your brain. When these levels increase, it may decrease your desire to eat or feelings of hunger. Given its effects on neurotransmitters, it is also sometimes prescribed for people with eating disorders like bulimia nervosa

 

Studies show that phentermine can help people lose 5-10% of their body weight within three months of prescribed use. However, losing closer to 5% of your body weight in the first few weeks of use is more common. 

 

Perhaps more intriguing is that studies show that phentermine may be more suitable to help when combined with topiramate, a medication (called Qsymia) that primarily treats seizures. However, it is also known to be an appetite suppressant. Compared to other commonly used weight loss medications, the combination of phentermine and topiramate was the most likely to help people lose the initial 5% of body weight.

 

Phentermine side effects and precautions for patients with hypothyroidism

 

Phentermine is safe for only short-term use, as we have no long-term studies to prove it is safe and effective. Yet, in combination with topiramate, it is FDA-approved for long-term use.

 

Severe adverse effects and reactions to phentermine are rare when used as prescribed. However, there are some known side effects in people who take the combination of phentermine and topiramate, including:

 

  • Dizziness
  • Heart palpitations
  • Flushing or redness of the skin
  • Sleep difficulties
  • Dry mouth 
  • Constipation

 

People taking other medications like levothyroxine need to be especially careful when taking phentermine. Because phentermine alters hormones like norepinephrine, it can strain your adrenal glands. The adrenal glands make norepinephrine and also work closely with the thyroid. 

 

Therefore, if you have an underactive thyroid, you will want to ensure your thyroid levels are normal with a complete thyroid blood test. Some patients with hypothyroidism who struggle with weight gain can take thyroid medication like levothyroxine along with phentermine, so long as a doctor carefully manages both medicines. 

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Also, some people may develop a tolerance to phentermine and may no longer have appetite suppression. These people will likely need to stop the medication if they become desensitized to the effects of phentermine.

 

How to achieve healthy weight loss with a thyroid condition

 

Weight gain is a common side effect of hypothyroidism. Excess weight can undoubtedly come from a slower metabolism from underactive thyroid. Still, the other symptoms of hypothyroidism like fatigue, depression, and joint pain can make it harder to get physically active. It can also be tough to stick with preparing healthy meals if you experience these symptoms.

 

Weight management with hypothyroidism requires several different steps, including:

  • Taking the correct dose of thyroid medication
  • Getting regular thyroid testing and follow-up with your thyroid doctor
  • Daily physical activity with moderate-intensity exercise at least three times per week
  • Healthy, well-balanced meals that are full of anti-inflammatory foods
  • Getting plenty of sleep each night
  • Reducing stress 


Weight gain from hypothyroidism can be tricky. It is often a combination of several factors that causes your waistline to increase. If you are struggling with weight loss and have hypothyroidism or Hashimoto’s disease, one of the first steps you will take is to test your thyroid and meet with your thyroid doctor.

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Julia Walker, RN, BSN

Clinical Nurse

Julia Walker, RN, BSN, is a clinical nurse specializing in helping patients with thyroid disorders. She holds a Bachelor of Science in Nursing from Regis University in Denver and a Bachelor of Arts in the History of Medicine from the University of Colorado-Boulder. She believes managing chronic illnesses requires a balance of medical interventions and lifestyle adjustments. Her background includes caring for patients in women’s health, critical care, pediatrics, allergy, and immunology.

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