Q & A: Does Feeding Decrease Absorption of L-Thyroxine?

I recently read one of your posts on your Animal Endocrine blog site regarding 'How Do We Treat Dogs with Hypothyroidism.' In that post, you indicated that thyroid replacement medication should be given on an empty stomach. And you indicated that this was quite important.

My question about this is: I have a number of dogs that are on L-thyroxine but the owners have always given the medication at the time of feeding. If we are doing post-pill serum T4 testing at 4 to 6 hours after administration of the morning dose and adjusting the daily dose according to those thyroid results, does it matter that the owners give the pills with food?

Thank you.

My Response:

Good question. Yes, the absorption of L-thyroxine (L-T4) in dogs has indeed been shown to be much higher when given on an empty stomach.

To my knowledge, this difference in the absorption has only been reported for Leventa, a liquid L-T4 medication (1). In one study, food intake concomitant with L-T4 oral administration delayed L-T4 absorption and decreased its rate and extent by about 45% (2).

I know of no other reports of the effects of feeding on absorption of L-T4 tablets in dogs, but I assume that the differences in absorption might also be true for other brands and formulations of L-T4.

The bottom line: In your patients, giving the L-T4 with food is not wrong. If the post-pill serum T4 is within the therapeutic range and the dog has responded with resolution of the clinical signs of hypothyroidism, I certainly wouldn't change the dose or feeding regime.

My point in that my L-T4 blog post is that if the owners did administer the L-T4 pill on an empty stomach, the daily required dose of L-T4 could likely be lowered. In some dogs, that could be a huge issue in the success of therapy, but in most dogs it probably doesn't make that much difference clinically.

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(2):95-101.