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What is the Connection Between Synthroid® and Phentermine?

By B. Chisholm
Updated Mar 03, 2024
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There is not much of a connection at all between Synthroid® and phentermine from either a pharmaceutical or medical perspective. The drugs are used to treat different conditions, contain different ingredients, and have completely separate dosing instructions. One thing that both have in common is that they can both facilitate weight loss in some patients, which has led some people to take the drugs together in the hopes of losing a lot of weight in a short amount of time. This practice is widely viewed as dangerous by the medical community, and mixing prescription drugs without the recommendation of a licensed healthcare provider is also illegal in many places. There aren’t usually any circumstances under which a person would be prescribed both drugs simultaneously, which makes the connection somewhat tenuous.

Main Functions of Each

Synthroid® and phentermine have very different indicated uses. Synthroid® is the trade name of a drug that is used to treat hypothyroidism, which is a condition marked by low thyroid levels. Although Synthroid® might cause weight loss in patients with low thyroid because of the way in which it normalizes thyroid levels, it is not indicated for use in people with normal thyroid levels and it isn’t intended for weight loss at all. Using it for weight loss might, in fact, be damaging.

Phentermine is a weight loss agent, but is contraindicated for use in patients with thyroid problems. It is indicated for use in severely obese patients, and it works primarily by suppressing the appetite. This drug has amphetamine-like qualities and is potentially addictive, so it should be used only in the short-term, typically to "kick-start" weight loss that will then be sustained and continued with other drugs and lifestyle changes. It might also have other excitatory effects, such as hypertension and restlessness.

In general, both drugs should be given only under strict medical supervision by a doctor who knows the patient's clinical history, any underlying diseases, and other medications. Both could interact with concomitant drugs and diseases. Once phentermine is stopped, normal appetite resumes, so it is vital for patients to institute dietary and lifestyle changes before this point. People who are unable to keep their weight down once off this drug sometimes look to other medical fixes, which may include Synthroid® — but most experts agree that this can be very dangerous.

Use for Thyroid Conditions

Synthroid® is manufactured exclusively for the treatment of hypothyroidism. Hypothyroidism is a condition that happens when the thyroid gland doesn’t produce enough thyroid hormone, and weight gain is one of the biggest side effects of this imbalance. Synthroid® contains levothyroxine, a synthetic form of the hormone, and it basically serves to re-balance or normalize the levels. As a result, the excess weight a person has gained because of the problem sometimes goes away once on the drug. It is important to note, though, that the drug itself doesn’t cause the weight loss. Rather, it’s the re-balancing of hormone levels that has that effect.

People who have normal thyroid levels do not need to replace thyroid hormone and should, therefore, not take Synthroid®. The hormone levels in the body are finely regulated and influence each other intricately. Taking a hormone that is not medically indicated might trigger a cascade of changes in the body with possibly dangerous results. As such, Synthroid® should be used only when medically indicated and under a doctor's supervision.

Tips for Weight Loss

Most experts agree that there is no “quick fix” for weight loss. Taking a combination of drugs, be they Synthroid® and phentermine or other drugs entirely, is not a safe or often even a very effective course. In most cases, the only way for a person to lose weight is by exercising more frequently, eating less, and choosing more wholesome foods. It often helps for people to have support while trying to lose weight, and getting the help of a dietitian or nutritional therapist or enrolling at a gym with personal trainers on staff can be useful, too.

Common Concerns

Anyone who worries that their weight gain may be related to low thyroid levels should usually see a healthcare provider for an exam and lab workup. At least from a medical standpoint, diagnosis and treatment of established hypothyroidism is simple. Similarly, people who are worried that their current weight loss plan isn’t working should get a professional opinion before attempting to self-medicate with unprescribed pharmaceuticals. Mixing drugs can be dangerous and potentially illegal, and can have a range of unintended consequences.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon992560 — On Sep 16, 2015

This article makes no sense. It is very common to take both since people who suffer hypothyroidism are normally overweight or obese. So it would make sense that people would use Adipex while on Synthroid. Who wrote this?

By Lostnfound — On May 27, 2014

I'm very leery of any weight loss pill. They all seem to have bad side effects, and if you can't take this medication if you have low thyroid, I don't know -- that just seems to indicate not many people should take it at all. I know it was the Phen-fen combination that was taken off the market years ago, so I'm wondering if any of these are really safe to take.

By Grivusangel — On May 26, 2014

It would figure that a drug that's worked for me for weight loss in the past is now not a possibility because I have low thyroid. Well, shoot. That stinks.

Phentermine worked very well for me for a good while and I would love to get back on it.

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