Treating Hypothyroidism Isn’t Always as Simple as it Seems

abstract & commentary

Synopsis: Calcium carbonate may interfere with the absorption of levothyroxine.

Source: Singh N, et al. JAMA 2000;283:2822-2825.

The effect of calcium carbohydrate on the absorption of levothyroxine is important since many postmenopausal women may be taking this combination of medications.

This study involved 20 hypothyroid patients (age range, 27-78 years; n = 11 men). All subjects had normal free T4 and thyrotropin levels before beginning the study. The subjects were instructed to take 1200 mg/d of elemental calcium carbonate along with their thyroxine for three months.

Levels of free T4, total T4, total triiodothyronine (T3), and thyrotropin were taken at baseline before taking calcium carbonate, at two and three months while taking calcium carbonate with their thyroxine, and two months after discontinuing the calcium carbonate.

Mean free T4 and total T4 levels were significantly reduced during the calcium carbonate period and increased when the calcium carbonate was discontinued. Mean free T4 levels were 1.3 ng/dL at baseline, 1.2 ng/dL during the calcium carbonate period, and 1.4 ng/dL after calcium carbonate was discontinued.

Total T4 levels followed a similar pattern. Mean thyrotropin levels increased significantly from 1.6 mL U/L at baseline to 2.7 mlU/L during the calcium carbonate period and decreased to 1.4 mL U/L after calcium discontinuation. Twenty percent of the patients had thyrotropin levels during the calcium carbonate period; the highest level was 7.8 mL/L. Mean T3 levels did not change.

The conclusion of the study was that calcium carbonate reduces T4 absorption and increases thyrotropin levels.

Comment by Ralph R. Hall, MD, FACP

Singh and colleagues also note that other drugs have been shown to interfere with the absorption of levothyroxin. These include sulfate, sucralfate, bile acid sequestrants, and aluminum hydroxide. High-fiber diets have also been shown to impair thyroxine absorption. Other drugs such as phenytoin (Dilantin), carbamazepine (Tegretol), and sertraline (Zoloft) may accelerate disposal.

It would have been interesting to see if thyroid function would continue to worsen had the calcium carbonate been given over a longer period of time. The liklihood is that there would have continued to be a worsening of the status of her thyroid function.

It behooves us to monitor all patients who are taking other medications with their thyroxine a little more often. Polypharmacy is complicated.