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About Excipients

An excipient is an inactive ingredient of medicine which is added in the formulation for a specific purpose. They must be listed on the medicine’s labelling and can also be found in the patient information leaflet (PIL) within the packaging.

Excipients are supposed to be inert but often they have been found to cause problems for patients. If you have new symptoms or a return of your hypothyroid symptoms and you have had a change of brand/generic medication, do consider that your symptoms may be due to adverse reactions to the excipients and consider asking to be given a brand/generic medication that better suits you.

Excipients are used for various reasons such as fillers, binders, adhesives, diluents, disintegrants, glidants, lubricants, flavourings, tablet coatings, colourants, preservatives and sweeteners. Here is a list of why some of them are used (this is only a few of the many used):


i.e. gelatin, glucose, lactose, cellulose derivatives (methylcellulose, ethylcellulose, hydroxypropyl methylcellulose, hydroxypropyl cellulose), starch, polyvinyl pyrrolidone (Povidone), sodium alginate, carboxymethylcellulose, acacia, etc. These bind the tablet ingredients together giving form and strength.


i.e. lactose, dextrin, glucose, sucrose, microcrystalline cellulose, sorbitol, dibasic calcium phosphate dihydrate, calcium sulphate dehydrate, etc. These provide bulk and enable accurate dosing of potent ingredients.


i.e. starch, cellulose derivatives and alginates and crospovidone. These aid dispersion of the tablet in the gastrointestinal tract, releasing the active ingredient and increasing the surface area for dissolution.


i.e. colloidal anhydrous silicon and other silica compounds, cornstarch, talc, etc. These improve the flow of powders during tablet manufacturing by reducing friction and adhesion.


i.e. stearic acid and its salts (e.g. magnesium stearate), talc and paraffin. These have similar actions to glidants, however, they may slow disintegration and dissolution. The properties of glidants and lubricants differ, although some compounds, such as starch and talc, have both actions.

Tablet coatings

i.e. sugar (sucrose) and polymers. Polymers that are insoluble in acid, e.g. cellulose acetate phthalate, are used for enteric coatings to delay the release of the active ingredient.


i.e. synthetic dyes and natural colours. These are used to improve acceptability to patients, aid identification and prevent counterfeiting as well as to increase the stability of light-sensitive drugs.

Do excipients in thyroid medication matter?

Thyroid UK is informed on a daily basis that patients have reactions to certain brands of levothyroxine, liothyronine and natural desiccated thyroid medicines. In most cases, when they change to a brand with different excipients, their symptoms disappear.

The case of Merck changing lactose to mannitol in their levothyroxine causing thousands of patients to become ill is an important one to remember. 

An article in the journal, General Cardiology, reported on a study “Inactive Ingredient in Oral Medications​1​ that explains about excipients and the most common adverse reactions. There doesn’t appear to be an up to date list of excipients but the Handbook of Pharmaceutical Excipients gives a lot of information.  

This 2019 article – The Excipients between Effects and the Side Effects​2​ by Mohammed Abdul-Mutalib Abdul-Bari – explains a lot about excipients and concludes that:

“As shown above, excipients have been used for a long time because of its benefit in enhancing drug effect, performance, solubility and other beneficial properties that make the drug more effective and safe.

On the other hand, the potential adverse effect of the excipients should not be underestimated and a periodic review and follow up of side effects that may appear and occur after a drug administration should not be limited to the active medication in the drug formula but also the excipient should be taken into account

We all, therefore, need to be aware that it could be the excipients that might be causing any symptoms when we start on a new brand of thyroid medication and not the active ingredients.”

Mohammed Abdul-Mutalib Abdul-Bari


  1. Pardini, PharmD C. Excipients in Medications May Trigger Adverse Reactions in Some Patients. Cardiology Adviser. Published April 15, 2019.
  2. Abdul-Mutalib Abdul-Bari M. The Excipients between Effects and the Side Effects. Journal of Medical Care Research and Review. Published May 27, 2019.

Date updated: 10.04.20 (V1.0)
Review date: 10.04.22

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