Clinical Briefs
With Comments from John La Puma, MD, FACP
Calcium Carbonate and Hypothyroidism
September 2000; Volume 3; 108
Source: Butner LE, et al. Calcium carbonate-induced hypothyroidism. Ann Intern Med 2000;132:595.
"A 49-year-old woman with a history of pseudotumor cerebri, hypothyroidism, and gastric bypass surgery was admitted for an episode of bradycardia and syncope while undergoing an upper gastrointestinal study. Upon further evaluation, the patient reported headache, dizziness, mood changes, depression, and lethargy. Hypothyroidism was suspected, and the thyroid-stimulating hormone (TSH) concentration measured on admission was 21.85 IU/ml (normal range, 0.35 to 5.5 IU/ml). The TSH concentration remained elevated upon repeated measurement, a finding that supported the diagnosis. Home medications included l-thyroxine, 150 mg/d, and calcium carbonate (TUMS®, SmithKline Beecham, Philadelphia, PA), three tablets daily, for osteoporosis prevention. Upon further questioning, the patient said that she ingested l-thyroxine and the calcium carbonate tablets simultaneously. The patient was informed of the possibility of a drug interaction and was advised to separate the administration of these medications. On the basis of this discovery, the patient was discharged with instructions to take the same l-thyroxine dosage and to have her TSH concentration measured as an outpatient. A follow-up TSH concentration recorded 8 months after discharge was 3.31 IU/ml (normal range, 0.46 to 8.10 IU/ml)."
COMMENT
These clinicians suspect that thyroxine was chelated with the calcium carbonate dosage taken at the same time, and therefore malabsorbed. This is not a half-life dependent effect, as T4 and T3 half-lives are measured in days.
Certainly gastric bypass patients malabsorb thyroid hormones, and have many other vitamin and mineral deficiencies. Whether clinicians need to be cautious just with people who have had jejunoileal bypass for obesity, or with everyone who takes calcium carbonate and thyroxine is what’s at stake. For those who cannot get their calcium through food and are hypothyroid, caution is needed: Vitamins and minerals in concentrated forms have drug-like effects.
Recommendation
Advise patients taking calcium supplements and supplemental thyroxine to take their pills at different times during the day.
September 2000; Volume 3; 108
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