How to Safely use Phentermine with Thyroid Problems

How to Safely use Phentermine with Thyroid Problems for Weight Loss

Phentermine CAN help with weight loss, but only if used correctly.

And it can get a little bit tricky if you have thyroid problems, hypothyroidism or Hashimoto's thyroiditis.

If these conditions are untreated and you take Phentermine you may be putting unneeded stress on your adrenals and thyroid which can make you feel worse.

But, if you use this medication correctly you CAN get results and it can help you with weight loss.

Celebrate weight loss phentermine

​It's certainly not a miracle drug that will result in dramatic weight loss, but it can be helpful for a variety of reasons which I will outline in this post. 

​I also want to show you how I use it on my patients, and how it can be MOST effective in balancing hormonal levels. 

Let's talk about how to use phentermine safely with thyroid problems...


How does Phentermine work?

First of all, you need to realize a few things: 

I don't recommend using weight loss drugs by themselves to try and force your body to lose weight because this tends to cause MORE problems than it helps.

If you will recall from my previous posts, in order to lose weight you NEED to balance your hormones.

If you are still of the opinion that weight loss is about reducing your calories then stop reading right now and read this post.

​Now that we are all on the same page we can move on...

Is this the first time you've heard of Phentermine before? ​

Let me fill you in on some basics...

Phentermine is a prescription medication frequently used for weight loss and it acts by doing 2 main things in your body: ​

​1. Increase your Basal Metabolic Rate (AKA boosts your metabolism)

2. Reduces your appetite ​(AKA you eat less!) 

These may sound good to you, but you have to be cautious especially if you have thyroid and/or adrenal problems. 

Phentermine acts to increase your metabolism by increasing the amount of norepinephrine (adrenaline) in your body (1).

​This same mechanism also results in a reduction in appetite and is the same mechanism that many ADD/ADHD medications use to help with focus and attention. 

Taking phentermine may cause patients to feel more energy, increase their basal metabolism and reduce their appetite.

Sounds pretty good, right?

Not so fast!

There are a few things you need to know if you have thyroid problems...

Phentermine and Your Thyroid + Adrenals

As you know, your thyroid (in conjunction with your adrenals) controls the metabolism of your body.

Patients with hypothyroidism usually have a lower than normal metabolism (2) (sometimes as much as 30-40% below normal!).

So they are already at a disadvantage when it comes to losing weight.

At first glance, you may think that adding Phentermine, which increases metabolism, would be a good idea.

Unfortunately, it isn't that easy.

Taking phentermine when your thyroid is untreated puts excess strain on your adrenals which can ultimately make thyroid function worse.

Remember that your thyroid gland and adrenals function as one unit (3)!

Thyroid adrenal reset complex 400 x 350

Because of this, I don't recommend taking Phentermine unless you are currently being adequately treated with Thyroid hormone and you have addressed your adrenals. 

For patients that see me in my office who have been untreated, I recommend they use thyroid medication with adrenal support for at least 4 weeks before adding phentermine. 

To see if your physiology can handle phentermine answer these questions:

Can your Thyroid handle Phentermine?

  • Is your basal body temperature < 97.8?
  • Are you taking T4 only medication like Synthroid or Levothyroxine?
  • Do you still have hypothyroid symptoms?
  • Do you have palpitations or feel like your heart is racing?

If you answered Yes to any of these questions then I don't recommend that you start taking Phentermine until these are addressed!

For many patients that means changing to a thyroid medication that contains T3 hormone - either NDT or Liothyronine. ​

Can your Adrenals handle Phentermine?​

  • Are you dependent upon taking Adrenal supplements like adrenal adaptogens or adrenal glandulars
  • Do you have symptoms of Adrenal fatigue: Wired but tired, salt and sugar cravings, decreased sleep, inability to tolerate stress?
  • Are you dependent upon caffeine for energy?

If you answered yes to any of these questions I don't recommend starting phentermine right away. 

You will want to address both your thyroid and adrenals prior to doing so!​

How I Use Phentermine in my Office

This is how I recommend using Phentermine to my patients:

As I said before, phentermine by itself doesn't actually balance your hormone levels and lead to lasting weight loss...

But it does have some interesting side effects that we can exploit to make balancing your hormones easier.

Currently, in my practice, I only recommend using Phentermine for patients who have one or more of the following diagnoses: 

  • Leptin Resistance
  • Insulin Resistance
  • Are using fasting protocols like Intermittent fasting or prolonged fasting

By far the most beneficial side effect of phentermine is its ability to reduce appetite

This side effect can be VERY helpful in patients who suffer from intense food cravings (like those with Leptin Resistance) or in patients who need to be on fasting protocols because of Insulin Resistance.

As you may know the key to reversing insulin resistance is reducing insulin levels, and there is no better way than fasting.

Once insulin levels drop low enough your body begins to burn purely fat cells as an energy source.

Burning fat in the presence of insulin

​To reverse these hormonal conditions I will have patients use phentermine ONLY on fasting days to curb their appetite and make the fasting process easier. 

Using phentermine in this way makes it so patients are only using it 1-2 days per week for intermittent fasting, or 2-4 days in a row for more prolonged fasting periods

Do you see the difference between using this medication by itself for weight loss and using this drug to balance other hormones for weight loss? ​

​There is a BIG difference!

How NOT to use Phentermine​

Unfortunately, the WRONG way to use phentermine is the most common way that it is used.

Most people believe that taking the medication will reduce their appetite which means they won't eat as much which means they will lose weight.

Remember that this does NOT lead to long-lasting weight loss and about 98% of people who try this type of diet ultimately end up gaining the weight back (4).

The biggest mistake you can make is using phentermine to reduce your calories for a prolonged period of time while still eating poor quality foods - just less of them.

How not to use phentermine

This WILL result in further thyroid damage (5) and weight gain once you stop the medication.

Beware of this trap!​

Traditional advice is to take the medication daily for several months, then stop it. I've even seen some patients on it for as long as 12 months.

If you are taking it long term (which I don't recommend) make sure that you only use it 5 days out of the week, and take a 1 month break every 2 months.

Doing this will allow your receptors to re-sensitize to the medication and keep it working for you longer. ​

Side Effects to Watch out for

Even though phentermine CAN be beneficial does not mean that it will be for every individual.

In my experience up to 15-20% of patients don't tolerate it well due to a variety of side effects. 

The most common side effects that I see in patients include the following:

  • Insomnia - This is a BIG one, do not take this medication if it keeps you from sleeping. Lack of sleep leads to weight gain (6)!
  • Worsening Fatigue - This is what happens when you take it without treating your adrenals or thyroid FIRST
  • Fatigue in the AM with bursts of Energy at night - Another sign that your Adrenals are not ready for this medication
  • Irritability
  • Dry mouth
  • Palpitations or racing heart
  • Hair loss

There are obviously more side effects and you can view the complete list here (7).

Phentermine side effects

If you experience these side effects you can try reducing the dose by half, or even taking the medication every other day (or only on fasting days!). 

But don't continue taking them if the symptoms don't resolve, it's not worth impairing other systems in your body to lose a couple of pounds. 

Don't take Phentermine without taking these Supplements

If you currently have Hypothyroidism or Hashimoto's and you are considering taking Phentermine, make sure you are taking an adrenal supplement. 

In my patients, I typically wait until they have been on adrenal support (AND thyroid support with T3 medications) for at least 4 weeks before I start phentermine.

Unfortunately, you can't control the thyroid medication that your doctor gives you, but you can control the adrenal supplements that you take. 

Adaptogenic herbs to use with phentermine

If you have low-grade adrenal fatigue I recommend starting with adrenal glandulars.

If you have SEVERE adrenal fatigue you may benefit from a combination of adrenal glandulars + adrenal adaptogens to boost adrenal function.

Combining these supplements with proper thyroid medication (like Naturethroid or liothyronine) balances both your cortisol and thyroid levels and puts the body in a position to burn fat like crazy. 

​I will often combine these therapies with a fasting protocol (to reduce insulin levels) with great results. 

​Wrapping it up

Phentermine isn't a magical medication that will help EVERY patient lose weight. 

Having said that it can be used to help balance other hormones in your body and ultimately lead to weight loss.

My favorite side effect of this medication is appetite suppression which can be used in conjunction with fasting to reverse insulin resistance and leptin resistance which will lead to lasting weight loss.

Even though it can be helpful in some people it can also cause more harm than good, especially if it's used in patients who are being undertreated for their thyroid or in patients with untreated adrenal fatigue.

That's why I don't recommend taking this medication unless you are also on thyroid medication and adrenal support.

Now it's your turn: 

Have you used phentermine to help balance your hormones?

Did it work for you long term?

​Would you recommend it to other thyroid patients?

Leave your questions or comments below!

References (Click to Expand)

Dr. Westin Childs

About Dr. Westin Childs

Hey! I'm Westin Childs D.O. (former Osteopathic Physician). I don't practice medicine anymore and instead specialize in helping people like YOU who have thyroid problems, hormone imbalances, and weight loss resistance. I love to write and share what I've learned over the years. I also happen to formulate the best supplements on the market (well, at least in my opinion!) and I'm proud to say that over 45,000+ people have used them over the last 4.5 years. You can read more about my own personal health journey and why I am so passionate about what I do here.

62 thoughts on “How to Safely use Phentermine with Thyroid Problems for Weight Loss”

  1. I so appreciate you and your blog
    I have a nodule on my thyroid I have every single symptom of thyroid and adrenal fatigue my blood test for thyroid issues come back in normal range over the past two years I have gained 30 pounds I am looking into taking Phentermine would you recommend that I do so even though I have every single symptom of thyroid disease my endocrinologist is just watching my symptoms but not treating them because my blood tests are coming back okay I appreciate you your time and your advice thank you peaceful day to you

    • Hey Sheshe,

      I don’t think using phentermine would be a good idea in your situation. If you were to take it now you would probably lose a few pounds here and there, but once you stopped it would most likely come right back. It is certainly NOT a long term weight loss solution by itself.

    • Just read this article and will read the previous one you recommended regarding Phentermine.
      I have a question. I was told my cortisol levels were extremely low. (Surprising since I’m a 45yo female that I feel I’ve been stressed since birth) so my endo dr. immediately put me on hydrocortisone pills. Would Phentermine not be recommended in my case for weight loss because of the low cortisol and the pills?

  2. Dr. Childs,

    I’ve been struggling for the past 10 with hypothyroidism, adrenal fatigue, and fibromyalgia. I used phenthermine in the past, which worked great while I was on it, but after 3 – 4 months, all the weight bounced back! So I decided not to used again, but my fatigue and exhaustion is driving me crazy! No doctors have figured it out yet how to help me.

    • Hi Libby,

      Great point. If you use phentermine incorrectly then all the weight that you lose while taking it will simply come back once you stop.

  3. This is a nice blog informing you about thyroid and phentermine. This is my question… I just started taking phentermine HCL 30 mg now and its been a week, but I also have Hashmito Thyroiditis and Graves. I have 1/3 of my thyroid left after surgery so I was wondering about the DFS to take with the phenthermine because of the fatigue and trying to exercise. I have been off thyroid medicine for over 2 years because it was over medicating me so I just take b-12 pills and biotin for energy. I need to lose weight so that was the purpose of going on phenthermine. Your thoughts.

    • Using phentermine in the conventional way will lead to some weight loss initially but it will all come back once you stop the medication.

  4. Dr. Child’s,

    First off, your website has been a great resource and I’ve learned a lot. I’m currently taking 3/4 grain Naturethroid and have been treated over the last 3 months with adaptegens to support my thyroid and adrenals. My question is this, if I was to do alternative day fasting (36 hours) and take the 30 mg phentermine I currently have at home, how far apart from this dose do I take the Naturethroid? Last I checked, my fasting insulin was 7.2 and I’d like to use intermittent fasting to bring that down. In addition, I’m eating a ketogenic diet of 20 or less carbs a day and test my blood ketones regularly to make sure I’m in ketosis. Even with this, my weight remains stable and I haven’t been able to lower that set point through the keto diet alone. But only by fasting. My guess is, once I can lower the fasting insulin through intermittent fasting, the weight will start to come off much easier. Your advice is appreciated!

    • I was wondering if the licorice root or Korean ginseng would help me with the phentermine for energy and weight loss. I have hashimoto and graves and 1/3 of my thyroid left but I don’t take thyroid meds since it was over medicating me

  5. I just had my fasting insulin done and it’s 7.1 and my leptin is 19.5 and A1C is 5.7…with this being the case, will fasting on the weekends with phentermine (Friday, Saturday, and Sunday) improve the leptin resistance? I understand fasting is great to improve insulin sensitivity but what does it do to leptin sensitivity?

  6. So I just started phentermine 18.75 mg day. I also take NDT for my hashimotos. I love the energy Phen gives me, but does any weight I lose on this automatically come back when I stop? I eat really healthy and exercise. I just haven’t heard of anyone keeping the weight off after stopping the pills.

  7. Hello,
    I was diagnosed with hyperthyroidism and anemia. Is it OK to take phentramine with those conditions? I have not lost any weight with the hyperthyroidism. Also, I am extremely dizzy. Is it OK to take? Thank you

    • Hey Liv,

      You should consult your primary care physician but in general I would suggest that it would not be a good idea to use phentermine with hyperthyroidism as that may put too much demand on your heart and lead to damage over time.

  8. My husband was on Phentermine for over 8 YEARS!!! He has now been off of it for at least 5 years but his body has never been the same. Initially he lost about 50 pounds and maintained that for the first few years. Then the weight just climbed back up (even on the med). He is now 48 years old and his body is incredibly resistant to weight loss. He is 5’10” and 210 pounds (most of the weight sits in his round distended abd).We have tried every form of healthy eating with NO change-Paleo, Ketogenic, well balanced, low calorie, etc. We see a functional medicine NP now but her knowledge seems limited with Phentermine (in fact their practice does prescribe it). My husband is currently being treated with Naturethroid, Leveothyroxine, Testosterone and Estrogen blocker. We have also tried lots of supplements, probiotics, allergy food testing, iGG, probiotics. Any suggestions on what we should do next? I have been unsuccessful in finding any studies on how to reverse the damage done by his long term phentermine use. Are there any you can direct me to so I could provide to our NP? I wonder if trying to support his adrenals would help. All of his glucose numbers are WNL but I think on the high side. Any suggestions would be greatly appreciated.

    • Hey Karen,

      Most functional medicine provider consider medications like phentermine to largely harmful long term (and some of that is warranted), but there are still ways to use the medication effectively without the damage that may result.

      It’s hard to say that his metabolic damage was caused purely by phentermine alone, especially if he has concomitant insulin resistance and hypothyroidism. I would recommend you look into fasting insulin levels (and if those are normal then 2 hour post prandial insulin levels). In addition I’m not really a huge fan of the combination of levothyroxine + NDT. It seems that he may have several hormone imbalances that are not being addressed correctly.

  9. Dr. Childs,

    IIn your experience, which is better for fat loss while using phentermine, extended fasting (i.e., 3 day fasts) or alternate day fasting? I’ve been fasting by having my last meal on Thursday evening and then taking a phentermine Friday and Saturday and then resuming eating a keto diet on Monday. I’ve been doing this since Thanksgiving weekend and have lost 13 pounds. What are your thoughts?

    • Hi Dawn,

      It really depends on your body, your appetite, your current metabolism and so on. Some people do well on extended fasts while others should avoid fasting completely. I usually make that decision based on a number of factors including past medical history, weight loss goal and so on. I wouldn’t say one is better than the other as a general rule but that one may be best for your body.

  10. I’ve been taking levothyroxin for 5 years now and my weight has been stuck between 133-140 every day it fluctuates by 4-5 lbs. A month ago my Dr finally prescribed phentermine for me because I was just stuck with my weight even with 45min to an hr of workout 5 times a week. My first week I finally got down to 131. Now I’m back at 133-137. I eat healthy and my portions are small……please tell me what I should do next.

  11. I have half of my thyroid the other half was removed due to a tumor the size of a tennis ball. I now have on the half of thyroid i have left two nodules one on the base, and one on the right side. In addition i also have a tumor on the right adrenal gland and a cyst on the left. I am allergic to synthriod and have been put on tirosint. I just went for lab work and my creanitine came back minor abnormal as well as my t3 and my acth. They were all just slightly higher than what they should be and i go back to my endo on may 23rd. I have been on adipex for the last year off and on and have managed to keep off 30 pounds. I am suppose to return to my PCP tomorrow to be put back on it. Any thoughts would be appreciated.

    • Hi Bonnie,

      In general you don’t want to stay on phentermine for an extended period of time as it may cause issues in cardiac tissue and so on. In cases such as yours it may be better to explore newer and more efficacious weight loss medications with your physician.

  12. I was diagnosed with hyperthyroidism can I take Phentermine or will my doctor prescribe Phentermine in this condition?

  13. Hi Dr Childs,
    My endocrinologist is considering prescribing Phentermine for weight loss, but I’m hesitant based on the many comments that the weight always comes back. I have Hashimoto’s (treated currently with levothyroxen and liothyronine) as well as insulin resistance (and take Metformin for that). Some years ago I was also diagnosed with leptin resistance as well. My naturopath has told me I have adrenal fatigue – I do take Ashwaganda.

    My question is – for long term weightloss, it sounds like you recommend intermittent fasting for the phentermine to work with thyroid issues – is that correct? Thank you!

    • Hi Rain,

      If phentermine is going to be used at all then that is probably the best way to use it – in an intermittent fashion. If you use it consistently each day to suppress your appetite then any weight you lose while taking it will come back once you stop.

      Hope this helps!

  14. Hi Dr Child’s

    I have Hypothyroidism and PCOS. I am being medicated for both. I wanted to know what the ideal dosage is for weight loss when you have hypothyroidism?

    Thank you

    • Hi Liv,

      I wouldn’t say that there is an “ideal” dosage for weight loss based on medical conditions but rather based on the individual and their sensitivity to the medication.

  15. Hi Dr. Childs,

    I’m 31 and found out earlier this year I have hypothyroidism. I’m so happy to have found your website, I can’t wait to read more! My PCP prescribed me 100MCG of Levothyroxine, which now shows my thyroid levels are normal.

    My question is: How do I tell if my adrenal glands are good to go? I have access to my blood test results if it’s on there I can look – or can they not be tested by blood tests?

    I want to start on Phentermine but want to make sure my adernal glands are okay and/or start taking the natural supplements to get them in order.

  16. Hi
    I was diagnosed with hyperthyroidism 5years a go and they did a radioactive iodine which made me hypo. I take medicine everyday and have my thyroid under control. My tsh is at 3.33
    My doctor gave me Phentermine, I only took it one day and had the worst headache I have ever had. I have not taken it again. I don’t know if maybe taken half would help me with the headaches. What do you recommend

  17. What a fabulous blog – very informative (and i have been searching for all sorts of information online). I have put on a tremendous amount of weight due to thyroid issues and have started on Levothyroxine – i need to try again in losing weight which has been incredibly difficult – could you suggest anything that i could take to help with the metabolism – I have a shaking machine now and will be increasing walks and exercise – I’m still working out the right dosage of my thyroid meds but would like to help my body get back to normal – can you advise me of anything??… Thanks again for putting this information out int he world it really is a help to those of us having to deal with these issues.

  18. I had a thyroidectomy in December 2017 due to cancer. I believe my endocrinologist has my hormones leveled out now at 175 mcg Synthroid and 5 mcg Cytomel, however I can not drop the weight. I was wondering if Phentermine might be an option for me?

  19. Hello Dr Childs,

    I’m a 30 year old female. Until the last six months I have always yo-yoed in weight. But since the death of my mother and a significant break up I have always found it easy enough to lose weight. I used Phentermine a year and a half ago loose 15kgs and it stayed off fine. This time when taking Adipex I seem to be putting on weight even with the drugs and all of a sudden have belly fat – which has never been a problem for me before. Is it possible that I am retaining water? Also – the Phentermine I was taking before was in the form of Duromine rather than Adipex and my Adipex dose is much lower.

    I’m very confused and concerned. I am eating well and walking for at least four hours a day and I just keep swelling. Any information would be very helpful. Thank you.


  20. What I would like to know is “when” to take the phentermine with regards to levothyroxine. I have Hashimoto’s, have been on levothyroxine many decades and always take it when I wake up. When do I take the phentermine?

  21. Hi,

    I am a 28-year-old woman, and I had my thyroid completely removed in 2014 since then I have gained about 40 pounds so my dr. recently prescribed me Phentermine to try to lose weight. Is there any advice you can give me while on this medication? I am currently taking levothyroxine 137 mg.

  22. I’ve been taking phentermine for about four months and recently found after going to my PCP for having to go potty so many times during the night. He did thyroid TSH test which resulted in a .230 (L) value with normal range being 0.450-4.500ulU/mL then a thyroid panel with all of those numbers being in range T4 7.7, T3 27, FTI 2.1 – could the phentermine be the cause or play into my TSH number being .230? I had an ultrasound on my thyroid yesterday and won’t know the results until next week. Thanks!

  23. Hi Dr Child’s,

    I’d like to take phentermine for 2 months. My S TSH is 0.30 and my S FT4 is 12.7. It’s been consistently like that for years and I have no thyroid symptoms. Would this drug be safe?


    • Hi Stella,

      It depends on how you would use it. If you use it daily to suppress your appetite then it probably will cause problems long term.

  24. Hello Dr. Childs,
    I am 24 years old and have hypothyroidism. I recently went to my doctor and he put me on levothyroxine and recommended me to take phentermine for the next 3 months until I see him again. I just started taking my levothyroxine, how long should I wait until I start taking the phentermine? thank you!

  25. I had hyperthyroidism, then my doctor treated me and gave me too much medicine when I got an iodine ablation. Now I have hypothyroidism and I’ve gained almost 45lbs. I take 125mg of Levothyroxine. My doctor prescribed me 37.5mg of Phentermine to help me lose weight. What’s the best way to take this medicine so that the weight doesn’t come back once I’m off of it, since I no longer have a thyroid?

  26. Hey, so currently I’ve been struggling with my weight. It was good until recently I’ve gained around 30 pounds. Next month, due to my religion I’ll be fasting for the whole month, meaning no food or water from sunrise till sunset. I have hypothyroidism and am currently taking levothyroxine. I really need to lose my weight, as it’s really affecting my self-esteem. Do you think taking phentermine in my case would help? Of course, when I do eat my meals, I will eat healthy foods to replenish my body and drink milk and water. I just really want something to work and don’t want to hurt my body or lose weight and gain it all back.

  27. Dr. Childs,

    I used Acxion in the past for a short period of time and it helped me lose some weight. Now that I’m older I’m having some under active thyroid issues and have been on Levothyroxine for a year now. I’m currently taking 77mcg and my thyroid level has been under control. But in the last year I have gained 15lbs that I cannot get rid of no matter what I do. So I started taking Acxion this week to help me with that. A 15mg dose is perfect for me, when I tried the 30mg it was too much. But wanted to know your opinion if its safe for me to take while taking Levothyroxine and what’s the best way of taking this medication.



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