Before I go any further, I must disclose: I do not use desiccated thyroid (Armour Thyroid, Forest Laboratories Inc.) in the management of hypothyroidism. Like most of my endocrinologist peers, I believe that desiccated thyroid is antiquated therapy and should no longer be used. Guidelines published by the American Association of Clinical Endocrinologists and other major endocrinology professional organizations support this position.
However, I am frequently asked by my patients about desiccated thyroid. Some of them tell me that a family member, friend, alternative care practitioner or other acquaintance has told them they do not believe in levothyroxine and advised them to be switched. This, along with several negative comments by patients on this blog about levothyroxine, is why I chose to write about this issue in detail now.
Desiccated thyroid is made from dried and powdered animal thyroid gland, a by-product of domesticated animals raised for the meat industry. For many years in the past, it had been successfully used in the management of hypothyroidism. However, once levothyroxine became available, desiccated thyroid fell out of favor. Recently, there has been resurgence in the use of desiccated thyroid as alternative medicine practitioners have proclaimed the benefits of natural over synthetic thyroid hormone preparations.
So why do I and most other endocrinologists refuse to use desiccated thyroid?
There are a number of reasons. First and foremost, desiccated thyroid preparations have an unacceptable level of variability batch to batch, often resulting in unacceptable variation in thyroid-stimulating hormone. The current USP standards specify that the amounts of levothyroxine and liothyronine in each 65 mg of desiccated thyroid should be 38 mcg and 9 mcg; however, the actual amounts vary considerably. According to the American Society of Health-System Pharmacists Big Red Book, the mean concentrations of levothyroxine and liothyronine in each 60 mg of desiccated thyroid ranged from 8.8 mcg to 59 mcg and 7.9 mcg to 18 mcg, respectively.
Part of the problem is that many manufacturers have used iodine content rather than actual thyroid hormone to standardize their preparations. Some manufacturers (ie, Armour Thyroid) perform bioassays to maximize batch-to-batch reproducibility. However, as noted above, the range of levothyroxine and liothyronine can vary considerably, even in products standardized by bioassay instead of iodine content.
I and many endocrinologists are concerned when the brand of levothyroxine is switched without our knowledge to other brands or from brand to generic. Whenever a patient must be switched from one levothyroxine product to another, we always recheck the TSH in several weeks to confirm the dose remains optimal. Even as little as a 10% difference between similarly labeled levothyroxine products can result in large variation in clinical response as measured by TSH. When managing my patients on levothyroxine, sometimes I change the dose by as little as an extra half pill more or less per week
If we consider slight variation between various levothyroxine products to be clinically important, then the much larger variation within desiccated thyroid preparations is unacceptable.
-To be continued-
AACE Thyroid Guidelines. Endocr Pract. 2002;8.
AHFS Drug Information. 68:36.04.