Synthroid/Levothyroxine Complete Overview – History, Usage, Dosage, Interactions, Side Effects

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The history of Synthroid, a medication for thyroid dysfunction in the form of levothyroxine sodium tablets, dates all the way back to the early 20th Century.

Thyroxine, levothyroxine at its purest form, was isolated by biochemist Edward Calvin Kendall at the Mayo Clinic in Rochester, Minnesota in 1914.

This experimentation first was conducted from the extracts of a hog’s thyroid glands. Then, in 1927, the hormone was created by two British chemists: George Barger and Charles Robert Harington.

Overtime, levothyroxine has been one of the most prescribed medications in the United States of America, when it comes to thyroid disorders.

Within the last 10 years, levothyroxine has become the 2nd most used prescription, with over 20 million filled prescriptions year-to-year.

Medical Use

Synthroid/levothyroxine has been used to cure hypothyroidism, or a low-functioning thyroid. Its goal is to add more thyroid hormones to the body. In turn, this should help the thyroid gland perform better – or how it normally should.

This substance needs to be used when the thyroid gland is either injured or showing lower hormone levels as normal thyroid hormones are important for physical and mental activity in our bodies, as well as fertility and preventing serious disorders such as thyroid cancer and/or goiters.


Dosage for levothyroxine depends on a few factors. The official guide for dosage can be found on Synthroid’s website.

The first factor is based on age. If you are younger, then you will probably be getting a smaller dose compared to older people.

The second factor depends of weight. The more you weigh, the higher dosage you will receive for levothyroxine.

Also, depending on how low your thyroid hormone levels are will also directly affect your dosage amount. The last two factors that affect dosage are: your response to treatment and lab results. Depending on how you react to treatment, your doctor will lower or raise your dosage.

For example, if you are taking a low dosage of levothyroxine, but your body is showing no reaction to the medication, your doctor may raise the dosage, or visa versa, dependent on each situation.

Lab results may shine more light on how much levothyroxine you should actually be taking, based on blood tests, etc. Patients should be taking this medication regularly, without stopping. For consistency, take the medication at the same time every day for best results.

If you have concerns while taking levothyroxine, do not quit cold turkey all together. Talking to your doctor before making changes is always a smart choice.

How to Take

Levothyroxine or Synthroid is usually taken orally through a pill, without food. Your doctor will likely ask you to take levothyroxine one time per day with one hour before food (breakfast preferably).

You should drink an 8 ounce glass of water with the medication, unless your doctor has other thoughts for how you should take it.

Capsule form should be swallowed whole without chewing on the drug.

But, for children or people that can’t swallow, tablet form can be used as an alternative. With these tablets, break it and crush the substance and mix with water (approximately 1 to 2 teaspoons). The substance should never be mixed with soy formula, when giving to infants or children.

Always discuss these matters with a pharmacist or a doctor before breaking down this substance to a more finite being.

Dosages will also depend on the type of levothyroxine brands available for you to take.

Brands should never be changed without the approved consent of your doctor. If your doctor does not know of your intent to change brands, a different choice could have negative effects when paired with different medications.

How It Works

You may be wondering what causes an underachieving thyroid in the first place, and how levothyroxine works.

Well, the thyroid is responsible for producing two hormones: T3 (triiodothyronine) and T4 (levothyroxine).

The thyroid gland’s job is to create more T4 than T3 in the body, even though the T3 hormone is usually much more active than the T4 hormone. The cool thing about the human body is the fact that the T3 hormone can actually be changed to the T4 hormone if it is needed. You can learn more about the thyroid feedback loop here.

If the body does not naturally fix the problem, this is where medication can come into play.

Other options to help the thyroid gland along involves Armour Thyroid or other naturally desiccated thyroid prescription medications. These are medicines that contain actual thyroids and hormones extracted from pigs. There are some people who prefer this type of medication a lot more than Levothyroxine or Synthroid. Click here for a complete overview of Armour Thyroid vs Synthroid.

Even though levothyroxine is a synthetic drug, it still mimics the all-natural T4 hormone. In rare cases, a patient may need to take more T3 hormones in the form of Cytomel.


There are a variety of brands to choose from when it comes to levothyroxine.

Make sure that you know the negative interactions if paired with other substances.

For example, specific medications like colestipol, iron, sodium polystyrene sulfonate, antacids, calcium supplements, and other substances can actually worsen the problem of an under-active thyroid, by lowering the hormones even more. If you are taking other prescriptions, make sure that you take them 4 hours apart from levothyroxine.

When drug interactions occur, this could lead to some horrible side effects or may change the entire make-up of the drug itself.

Your doctor should have a list of things you should not mix with levothyroxine like prescription drugs, herbal supplements, and/or non-prescription products.

Some products that may interactive negatively with levothyroxine include, but are not limited to, digoxin, blood thinners, and/or sucroferric oxyhydroxide.

Side Effects

Similar to a lot of prescription medications these days, there are a substantial number of side effects that may occur with levothyroxine if the drug is misused or the body does not react well with the substance.

If the body still continues to struggle with low hormone levels with the thyroid, even after taking levothyroxine, symptoms may present themselves such as aching muscles, weight gain, dry skin, a slower heartbeat, and tiredness, among other things. Your doctor should be notified if these symptoms persist. They will ultimately have the final say on judging if the side effects outweigh the benefits or not.

Other negative side effects may include hair loss with levothyroxine. Usually, this only happens as the body tries to adapt to the medication, but hair loss may continue overtime for a more prolonged issue.

These are times when the body does not adapt and serious health effects happen when the thyroid hormone levels are higher than normal. These symptoms are much more prevalent in the form of sweating, shaking, mood changes, and headaches. Medical attention needs to get involved if effects like chest pain, swelling, seizures, or an irregular heartbeat happens within the body.

In a rare situation, an allergic reaction can take place when using levothyroxine.

If this happens, you may notice symptoms such as itching, a rash, trouble breathing, and dizziness. Call a doctor immediately and report these negative effects to the Food and Drug Administration (FDA). Their phone number is 1-800-FDA-1088, and their website is


When it comes to Synthroid, the controversies come in the form of regulations and rules.

Synthroid, itself, has been used for the last 50+ years on the market to treat thyroid disorders. This should definitely be reassuring to hear for people being prescribed this medication, as generally Synthroid has had a positive profile, while being used by millions of people, young and old.

One of the controversies Synthroid has had, involves a task force which was created for adult hypothyroidism.

This group included members from the American Thyroid Association (ATA) and the American Association of Clinical Endocrinologists (AACE). They published a joint endeavor in 2012 entitled Clinical Practice Guidelines for Hypothyroidism in Adults.

This was created to help with diagnosis and treatment for hypothyroidism and pushed Synthroid as the treatment option.

The controversy began to present itself when members of the task force were linked to the company that made Synthroid. This gave off a bad vibe seeming the committee only wanted to write a report that would further their financial ties.

With their guidelines in place, the committee concluded three things should happen when dealing with hypothyroidism.

First, a Thyroid Stimulating Hormone, or TSH, is the screening test that should be used when dealing with dysfunction of the thyroid. Secondly, T4 treatment is the best replacement option. Lastly, treatment for hypothyroidism should be based on each patient. The first two guidelines resulted in controversy.

The first controversy came with TSH being the primary testing for thyroid dysfunction.

The TSH test has been accused of not being accurate when helping doctors’ diagnose a thyroid malfunction. For example, Richard Shames, a doctor based in California, has written a variety of books on the thyroid. He believes the samples taken for a TSH test are not accurate because samples are usually taken in the morning, while the blood is not ran through a machine until the evening.

This is ample time for the hormones to slow down even more, resulting in an under-active thyroid diagnosis from doctors. For best results, many experts recommend testing for TSH as well as free T3, free T4, reverse T3, anti-TPO antibody, antithyroglobulin antibody and SHBG.

The second controversy came from the guidelines saying that T4 hormones are the best replacement option when treating a thyroid disorder.

In fact, this recommendation has pushed doctors to not use T3 hormones to help with the problem. This ignores a variety of studies that have encouraged doctors to use T3 and T4 hormones in unison to help with the problem.

A 2009 study, that was published in the European Journal of Endocrinology, proved that using T3 and T4 hormones together in therapy against a thyroid disorder had better results than treatments that were only levothyroxine-based for some patients.

These results included less depression and anxiety within their patients. That is not to say that Synthroid is not a good option, but for some patients, other options as be better. At the same time, for some people Synthroid will be the best choice for their body.

  1. Reply
    Amandeep singh August 8, 2021 at 9:44 am

    Result =3.31

  2. Reply
    Jan March 6, 2020 at 11:56 pm

    I’m so excited that I found this website. It’s loaded with helpful information. It’s obvious that the team behind it has done an extensive amount of research. I’m really impressed.

    For many years, I was hypothyroid and was on levoxyl. I stupidly ignored the medical advice to work with my endocrinologist to monitor my levels when I began earthing. [ ]

    In a relatively short period of time, I started to feel hyperthyroid each time I took my medication, a side-effect that the medical experts warned often happens. That led to an incredible journey (too long to go into here) that resulted in a struggle to get off the medication, all because I was foolish not to follow their advice. Fortunately though, after all was said and done, the grounding helped level out my thyroid.

    Earthing is an amazing, natural way to help (not cure) thyroid levels but it should be done carefully under a doctor’s care. But it really helps to educate the doctor if he/she isn’t familiar with earthing. When I did consult with another endocrinologist, it was tremendously helpful to print out the studies and research that is posted on the earthing website. It provides the doctor with information of what it is and what to expect. Earthing is a great anti-inflammatory practice which can be achieved by walking barefoot on the earth. There are also products that are sold if going barefoot outside isn’t possible.

  3. Reply
    Anna April 2, 2018 at 2:01 pm

    Does any woman on here have a severe menopause? If so, does thyroid medication help with that? I take ERT- large doses but have always had severe meno symptoms from the day of my hysterectomy which was 13 years ago. Shortly after the hysterectomy- I developed cysts all over my thyroid gland. Nothing was done but to watch and recheck at some point. It never got rechecked. Weird thing too- my fine , straight hair turned wavy and curly the last two years. Anything to do with thyroid function?.

  4. Reply
    Gertrude January 29, 2018 at 5:40 pm

    Karen I am sorry for your loss.I understand what you are going thru with the loss of your husband . Every one’s grieving is different. I lost my husband 19 years ago it is a very difficult time in our life. I can say what helped me was to keep my self busy I got involved with a group called widows group. they were going thru the same thing as i was, our grieving was different but we each understood. Time is different for every one. Yes it will help you if you if occupy your mind on feeling better and working toward a healthy life. Best of luck . God bless you

  5. Reply
    Karen December 27, 2017 at 1:54 am

    Hi Thyroid Advisor. I am a 66 yr old female who has always struggled w/ weight gain. In 1998 I was diagnosed w/ hypothyroidism. Currently I am on 125 mcg of levothyroxine. In 2014-2015 I managed to lose almost 60 lbs. on that diet known as the Nutrimost system. In short, 600-800 calories a day, for 40 days with all kinds of drops, etc. Then gradually more foods were added. This program promised to change your metabolism for good.

    I was doing very well keeping the weight off, then the bottom dropped out of my world, and changed drastically for the worse, in July 2016, my beloved husband of 34 years passed away suddenly. I am still grieving deeply for him, as he was the center of my life & soul mate. You guessed it – I started gaining weight again, and at one point in February 2017, my TSH numbers went from 2.37 to about 6.50. I never had such a drastic fluctuation, and my Dr increased the meds to the present dosage, and the TSH levels went back down to about 2.50. I read where intense grief can alter your hormone levels, and alter your metabolism, and of course – cause weight gain. I admit I have been overeating, sometimes not even realizing what I am eating, maybe stuffing down this terrible depression & sadness. And of course a “friend” bluntly observed that “you put all the weight back on”, and she has been impatient with my grieving – thinks I should be over him by now. I need to try & start losing weight again – just to feel better physically – this grieving has nearly killed me, but I want to be healthier if God intends to keep me here longer. It’s really going to be hard at this stage in my life, but I want to feel halfway decent again – energy wise, I am always exhausted & low energy.

  6. Reply
    Chelsea H. September 14, 2017 at 7:52 pm

    My mom has been taking Levothyroxine for about a year now and in June her doctor upped her dosage to 175mcg and ever since she’s been really achey all over her body and she says in hurts to do basic everyday things such as get out of bed and get dressed. Also recently her foot has started to peel a lot and she has tender new skin now and it hurts to walk. Could she possibly be overdosing, or should she change over to a different brand such as Synthroid?

    • Reply
      Thyroid Advisor September 27, 2017 at 2:51 am

      Hi Chelsea,

      Obviously, it sounds like something is not right. Could be overdosing, or a combination of both. Best to talk to your doctor about it and try different prescriptions and dosages. This is not normal!

  7. Reply
    Linda July 23, 2017 at 6:39 pm

    I think I have been gaining some weight while taking synthroid which is the opposite of what I want to do! What are the reasons behind this and what can I do to not gain anymore ?

    • Reply
      Thyroid Advisor July 25, 2017 at 7:21 pm

      Hi Linda,

      That is a relatively common ‘side effect’ of synthroid. Unfortunately the answer is not very simple, like most things in life. The cause is dependent on the person taking the drug and could be a multitude of reasons: incorrect dosage, improper diet or absorption, or even the drug itself. Another reason could be that synthroid is a T4 only medication. If you have difficulty converting the T4 to T3 hormones, weight gain can be expected.

  8. Reply
    Kelly July 21, 2017 at 8:44 pm

    Is there any type of levothyroxine brand you recommend? Like synthroid or something else

    • Reply
      Thyroid Advisor July 22, 2017 at 1:02 am

      Hi Kelly,

      There are differences between levothyroxine brands. Some have fillers or other additives that may work better or even worse for you. Even some of the generics from different stores have deviations. Most of them are small though. I doubt most would feel a real change, but it is there. If you are unhappy with your current brand, it might be good to change it up.

  9. Reply
    Anna July 20, 2017 at 10:55 pm

    how long after you start taking synthroid or levothyroxine do you feel better and less tired ?

    • Reply
      Thyroid Advisor July 21, 2017 at 1:00 am

      It largely depends on your dosage, but 2-4 weeks is the general period of time before you would start feeling improvements.

  10. Reply
    Carlie July 19, 2017 at 9:42 pm

    Can you take Synthroid with other thyroid support supplements like those on this website?

    • Reply
      Thyroid Advisor July 20, 2017 at 4:50 pm

      Hi Carlie,

      There are some vitamins, minerals, and herbs that can interact with Synthroid. I highly doubt it would be dangerous to your health, but it could interfere with the effectiveness of Synthroid/Levothyroxine, as we talked about in the article.

      Spacing them out 4 hours would be a better idea. It’s always good to ask your doctor as well, as they probably know more about your medical history and your goals.

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