Levothyroxine Ingredient List: What’s Actually Inside

Levothyroxine is consistently one of the most commonly prescribed medications in the United States beating out medications for common conditions like high blood pressure and cholesterol every single year. 

And yet, many people taking it have no idea what they are getting. 

Today that changes because you’re going to learn exactly what each dose of levothyroxine contains and how these ingredients may impact whether or not it’s working for you. 

Active Ingredient List

All prescription medications contain two different sets of ingredients: 

Active ingredients and inactive ingredients. 

Active ingredients are ingredients that exert a biological impact and impart some benefit (or drawback, as the case may be). 

When talking about levothyroxine, the active ingredient found in this medication (all forms and doses, by the way) is thyroxine also known as T4. 

Thyroxine is a bio-identical replica of the same thyroid hormone that your thyroid gland would produce if it was capable of doing so. 

Obviously, if you are taking levothyroxine this means that your body is unable to produce it which is why this medication was prescribed to you. 

The amount of thyroxine varies depending on the strength of the tablet you are taking. 

Levothyroxine can be prescribed in the following strengths: 

• 25 mcg
• 50 mcg
• 75 mcg
• 88 mcg
• 100 mcg
• 112 mcg
• 125 mcg
• 137 mcg
• 150 mcg
• 175 mcg
• 200 mcg
• And 300 mcg

The mcg dose correlates to the exact amount of thyroxine that is contained in each levothyroxine prescription. 

Most people taking levothyroxine will require a dose somewhere around 75 mcg to 125 mcg because that’s approximately how much the healthy thyroid gland produces (1) on a daily basis. 

But you may find that you require a higher dose depending on various factors including your genetics and your ability to absorb your medication. 

Inactive Ingredient List

The inactive ingredients found in any medication, including levothyroxine, are usually there to support the active ingredients, change the shape or color of the capsule (to prevent confusion among patients and doctors), and act as lubricants during the manufacturing process. 

These ingredients are generally felt to be less important than active ingredients but it’s a mistake to ignore them because they can and do cause problems for thyroid patients. 

The list of inactive ingredients varies based on the strength of levothyroxine that you are taking, but, regardless of what strength you are taking, they all contain the following: 

• Magnesium stearate – This is used as an emulsifier, binder, and lubricant to aid in the manufacturing process. There are no studies to indicate that magnesium stearate causes harm. 
• Microcrystalline cellulose – Commonly referred to as wood pulp or MCC, this ingredient is used to absorb moisture and to aid in tablet disintegration. MCC is felt to be quite safe (2) and is found in both over-the-counter supplements and medications but some people have reported mild side effects when taking it. 
• Colloidal silicon dioxide – This compound is used as an anti-caking agent that allows powder to freely flow during the manufacturing process. When taken in small amounts (and not inhaled) it’s considered to be safe. 
• Sodium starch glycolate – This compound is considered a super-disintegrant (3) which is included in levothyroxine formulations to aid in the absorption of the active ingredient (thyroxine). It is considered non-hazardous but its sourcing may be a problem for some thyroid patients as it’s created from rice, potato, or corn. 

In addition to these inactive ingredients, color additives are used to differentiate the various strengths (4) which are listed below: 

• 25 mcg tablet – FD&C Yellow No. 6 Aluminum Lake
• 50 mcg tablet – None
• 75 mcg tablet – FD&C Blue No. 2 Aluminum Lake, D&C Red No. 27 Aluminum Lake
• 88 mcg tablet – FD&C Blue No. 1 Aluminum Lake, D&C Yellow No. 10 Aluminum Lake, D&C Red No. 30 Aluminum Lake 
• 100 mcg tablet – D&C Yellow No. 10 Aluminum Lake, D&C Red Lake Blend (D&C Red No. 27 Lake and D&C Red No. 30 Lake) 
• 112 mcg tablet – D&C Red No. 27 Aluminum Lake, D&C Red No. 30 Aluminum Lake
• 125 mcg tablet – FD&C Yellow No. 6 Aluminum Lake, FD&C Red No. 40 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake 
• 137 mcg tablet – FD&C Blue No. 1 Aluminum Lake 
• 150 mcg tablet – FD&C Blue No. 2 Aluminum Lake 
• 175 mcg tablet – D&C Red No. 27 Aluminum Lake, D&C Red No. 30 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake 
• 200 mcg tablet – D&C Yellow No. 10 Aluminum Lake, D&C Red No. 27 Aluminum Lake 
• 300 mcg tablet – D&C Yellow No. 10 Aluminum Lake, FD&C Yellow No. 6 Aluminum Lake, FD&C Blue No. 1 Aluminum Lake 

While these inactive ingredients are included in very small amounts, they can cause problems for some sensitive thyroid patients. 

This is why some people prefer to use the 50 mcg levothyroxine tablet (referred to as the 50 mcg levothyroxine hack) to exclude as many extra color additives as possible. 

Other Important Information About Levothyroxine You Should Know 

On top of the inactive and active ingredients, there’s some additional information that you should know if you are taking levothyroxine: 

#1. Levothyroxine Contains Iodine

This may come as a shock to many thyroid patients but all forms of thyroid medications, including all doses of levothyroxine, contain iodine.  

This is because iodine is naturally found in all thyroid hormones, including thyroxine, and your body can break down and recycle the iodine found in this hormone (5) after you consume it. 

The exact amount of iodine extracted from each dose of thyroid medication varies depending on many factors but you can assume that each dose of levothyroxine provides 40-50% of its mcg dose as iodine. 

In other words, if you are taking 100 mcg of levothyroxine, you will be increasing your total body iodine status by 40-50 mcg with each dose. 

Given that the average person should consume 150 to 300 mcg of iodine per day, this represents a fairly substantial amount and should be taken into account. 

#2. Synthroid and Levothyroxine are not the Same and Contain Different Inactive Ingredients

Even though these medications are frequently conflated with one another, they are not the same and should not be considered direct substitutes for each other. 

Synthroid is a brand and trademarked name for thyroxine that is owned and manufactured by AbbVie Pharmaceuticals. 

Levothyroxine, on the other hand, is considered its generic counterpart. 

Given that it’s less expensive and covered by most insurances, it’s more commonly prescribed than Synthroid. 

Both of these medications contain equivalent doses of active ingredients but their inactive ingredient profiles differ slightly. 

For comparison, Synthroid contains the following inactive ingredients (6): 

• Acacia
• Confectioner’s sugar (corn starch)
• Lactose monohydrate
• Magnesium stearate
• Povidone
• Talc
• Additional color additives depending on strength (similar to levothyroxine)

This slight difference in ingredients is enough to make a big difference for certain patients with studies suggesting some individuals respond better to one over the other (7). 

In other words, if one of these medications isn’t working well for you, you may find success in swapping to another. 

#3. Levothyroxine May Not Work for Everyone

As far as thyroid medications are concerned, levothyroxine has a bad reputation as a medication that doesn’t work very well. 

Some of this bad reputation is founded in reality and some of it is not. 

Here’s what you need to know about levothyroxine and how it compares to other thyroid medications: 

• It’s considered synthetic but bio-identical – The term synthetic refers to sourcing and the term bio-identical refers to structure. Levothyroxine is considered synthetic because it’s created in a laboratory (it doesn’t come from nature). But it is also considered bio-identical because its structure is identical to the same thyroxine that the healthy thyroid gland produces. The fact that levothyroxine is synthetic does not automatically mean that it is inferior or bad as some people assume. 
• It contains only T4 thyroid hormone – T4 thyroid hormone is used as a pro-hormone in the body that doesn’t have any action until it is activated to T3. Other thyroid medications are considered more powerful than levothyroxine because they do not require this activation. Medications in this group include Cytomel and liothyronine. These medications can be used in conjunction with levothyroxine. 
• Whether or not it works depends on how much you are taking and how you are reacting to the inactive ingredients – You will know if you are taking enough thyroid medication because your thyroid-related symptoms will resolve. If you remain symptomatic despite taking levothyroxine then you may need to adjust your dose upward (or downward). If you still remain symptomatic after making dose adjustments then this means you may be experiencing problems related to the inactive ingredients which may warrant a switch to another type of thyroid medication. 

Levothyroxine Alternatives

The good news is that if you feel levothyroxine is not the best thyroid medication for you, you have plenty of other options to choose from. 

Here are four levothyroxine (T4-only) alternatives that may work better than levothyroxine: 

• Synthroid – As discussed previously, Synthroid is the brand-name alternative to levothyroxine and contains a slightly different ingredient profile. 
• Levoxyl – Is another brand name T4-only thyroid medication that is considered gluten-free and lactose-free (8). Synthroid is also considered gluten-free (9) but it does contain lactose. 
• Tirosint – Tirosint comes in a gel cap and contains only 3 inactive ingredients (10) making it much cleaner compared to Synthroid, levothyroxine, and levoxyl. Unfortunately, Tirosint is more expensive compared to levothyroxine but there are now generics available. 
• Tirosint-Sol – Tirosint-Sol is the only liquid thyroid medication on the market and contains the fewest inactive ingredients (glycerol and water (11)) making it the cleanest levothyroxine alternative available. Unfortunately, it’s also expensive and there are no generic versions available at this time. 

Believe it or not, there are even more thyroid medications beyond this list that you can look into as well that differ in their thyroid hormone content as well as their inactive ingredient list. 

While this may seem overwhelming if you are only used to taking levothyroxine, this is great news because it means there are plenty of options for you to choose from. 

The bottom line? You can find complete symptomatic relief as a thyroid patient but getting there may require some trial and error and using different thyroid hormones and different thyroid medications.