Alternative Dosage Forms of L-Thyroxine for Hypothyroid Dogs

One of my patients is a large-breed dog suffering with hypothyroidism. Unfortunately, the owner can not administer the L-thyroxine tablets to the dog. The dog is able to expel everything the owners forces in its mouth, and he is too smart to take the pills when hidden in food or given in a pill pocket.

Are there any injectable L-T4 preparations that are available? If it is available, what brand is best? Would transdermal application of L-T4 work?

My Response:

First of all, can they put the L-T4 tablet into a pill pocket?  It's hard to believe that the dog will not eat the tablet if put into a meatball or piece of cheese!

If not, Merke Animal Health makes a liquid L-T4 product called Levanta solution that may be useful in this dog (1). Studies of the pharmacokinetics of this L-T4 preparation indicate that once daily administration is adequate (2). If the owners can't put the liquid directly into the dog' mouth, they could try adding the dose to his food.

If the dog doesn't accept the Levanta, a compounding pharmacy could prepare a flavored suspension of oral L-thyroxine.  For example, one on-line pharmacy offers a levothyroxine oral oil suspension in a wide variety of flavors, including: anchovy; apple; bacon; banana; beef; beef/chicken; chicken-marshmallow; double bacon; double beef; double chicken; double fish; duck; half bacon; lime; molasses; orange; peppermint; spearmint; strawberry; strawberry/marshmallow; tangerine; triple chicken; triple-fish; tutti fruitti; vanilla/marshmallow and venison. From this list, there should be something that will taste good to your dog.

An injectable form of L-T4 is indeed produced, available as a single-use vial containing either 100 μg or 500 μg of Levothyroxine sodium for injection (3). However, this product is intended as an emergency treatment for myxedema coma, not for long-term use (4). This preparation would be rather expensive, and it would likely be cost-prohibitive. So I wouldn't recommend it for long-term L-T4 replacement of this dog.

Unfortunately, transdermal application of L-T4 just doesn't work well at all to normalize serum T4 values. Results of two studies found that the hormone is not absorbed into the systemic circulation to have an effect on all tissues of the body (5,6). In support of that, of the few dogs that I have evaluated on transdermal L-T4, none have responded to the transdermal replacement therapy with a rise in serum T4 concentrations.

References:

1. Leventa (levothyroxine sodium) oral solution. Intervet/Schering-Plough Animal Health. Package insert available at: http://www.leventa.com/default.asp
2. Le Traon G, Burgaud S, Horspool LJ. Pharmacokinetics of total thyroxine in dogs after administration of an oral solution of levothyroxine sodium. J Vet Pharmacol Ther 2008;31:95-101.  
3. Levothyroxine Sodium for Injection. APP Pharmaceuticals . Package insert available at: http://editor.apppharma.com/PIs/Levothyroxine_for_Inj_45804C_Jan_08.pdf
4. Pullen WH, Hess RS. Hypothyroid dogs treated with intravenous levothyroxine. J Vet Intern Med 2006;20:32-37. 
5. Padula C, Pappani A, Santi P. In vitro permeation of levothyroxine across the skin. Int J Pharm 2008 12;349:161-165.
6. Padula C, Nicoli S, Santi P. Innovative formulations for the delivery of levothyroxine to the skin. Int J Pharm 2009;372:12-16.