At last, they have caught up with us...
Interestingly, the testing was done Following an overnight fast, and they did Total T4 testing (rather than TSH and/or Free T4).
Concurrent Milk Ingestion Decreases Oral Levothyroxine Absorption
Number: LB SUN 25
Category: Thyroid/HPT Axis
Deborah Chon MD
Deborah Chon*1, Tamar Reisman2, Jane Weinreb3, Jerome M Hershman1 and Angela M. Leung4
1UCLA David Geffen School of Medicine; VA Greater Los Angeles Healthcare System, Los Angeles, CA, 2UCLA David Geffen School of Medicine; VA Greater Los Angeles Healthcare System, 3David Geffen School of Medicine at UCLA; VA Greater Los Angeles Healthcare System, Los Angeles, CA, 4UCLA David Geffen School of Medicine, Los Angeles, CA
Background: Levothyroxine is used for the physiologic replacement of thyroid hormone in patients with hypothyroidism and for serum TSH suppression in patients with thyroid cancer. In 2011, levothyroxine was the second most commonly prescribed medication in the United States, and frequent dose adjustments have been demonstrated to be a costly burden to the national healthcare system (1-2). Several studies have shown that certain foods and medications, such as calcium supplements, can interfere with levothyroxine absorption (3-5). There are no published studies specifically investigating the effect of cow’s milk, a common breakfast staple, on the absorption of levothyroxine. Cow’s milk contains approximately 450 mg of elemental calcium per 12 oz. serving.
Materials and Methods: To determine the possible effect of cow’s milk ingestion, we measured levothyroxine absorption with and without concurrent milk consumption. Pharmacokinetic studies were conducted in healthy adults without allergies to milk or levothyroxine, and who were not pregnant nor using oral contraceptives. All subjects had no history of known thyroid disease and a normal serum TSH concentration at baseline. Following an overnight fast, serum total thyroxine T4 (TT4) concentrations were measured at baseline and at 1, 2, 4, and 6 hours after ingestion of 1,000 μg of oral levothyroxine alone or when co-administered with 12 oz. of milk (2% fat). There was a four-week washout period between the two study visits.
Results: Ten subjects (mean age 33.7±10.2 years, 60% male) completed the study. The serum total T4 absorption over six hours, calculated as area under the curve (AUC), was significantly lower in those who consumed cow’s milk concurrently with levothyroxine, compared to those who took the levothyroxine alone (mean±SD: 67.26±12.13 vs.73.48±16.96; p = 0.02).
Conclusions: This is the first study to demonstrate that concurrent cow’s milk ingestion reduces oral levothyroxine absorption. The findings support previous literature showing the interference of elemental calcium with oral levothyroxine absorption. Patients managed with thyroid hormone replacement therapy should be advised to avoid taking levothyroxine simultaneously with cow’s milk.
[ Added after some extra thought... ]
We so obviously need to know whether the impact is similar for doses more typical of people actually taking levothyroxine every day - say 100 to 200 micrograms.
We also need to know the impact of the cereal content of so many breakfast cereals (whether wheat, oats, rice, or whatever else), the fortification of breakfast cereals by iron, vitamins, etc., and the huge level of sugar often taken in or added to the cereal. This needs to be looked at along with the the milk.
It is also so very important to realise that the effect of milk might be very different in those who are hypothyroid.
Of course, trials need to be done to isolate the impacts - such as this. We also desperately need real world trials to see what effects there are.