Calcium carbonate and L-thyroxine absorption in hypothyroid women
What it found
Taking calcium carbonate within two hours of thyroid medicine raised TSH levels, cholesterol, blood sugar, and blood pressure in postmenopausal women.
Moving the calcium supplement six to eight hours later brought those measures back down.
What they found
Studies in people
Thyroid hormone levels
When women took calcium within two hours of their thyroid pill, average TSH levels rose to 3.33 mU/L. That was higher than the 1.93 or 2.16 mU/L seen when calcium was taken further apart.
Elevated TSH rates
In the close-together schedule, 18% of women had TSH above 4.12 mU/L. None did when the doses were spaced out.
Cholesterol and blood sugar
Total cholesterol and fasting blood sugar were higher when calcium was taken near the thyroid pill. Spacing them out lowered both measures.
Blood pressure readings
Both top and bottom blood pressure numbers went up when calcium was taken close to the thyroid medicine. Separating the doses brought them back down.
TSH linked to other measures
For every 1.0 mU/L rise in TSH between 0.85 and 6.9 mU/L, cholesterol rose by 12.1 mg/dL and blood sugar by 3.12 mg/dL. Top and bottom blood pressure rose by 2.31 and 2.0 mmHg respectively.
What the authors conclude
- They advise that doctors should look beyond just TSH levels when checking patients who take medicines that reduce L-thyroxine absorption.
- The authors recommend also tracking cholesterol, blood sugar, and blood pressure in these patients.
How it was done
Calcium carbonate was already known to interfere with how well the body takes in L-thyroxine tablets. The authors wanted to measure how much this interference affects thyroid hormone levels and related health markers.
They followed postmenopausal women treated for an underactive thyroid who added 600-1000 mg per day of calcium supplements. They compared three timing schedules over several years: taking calcium long after the thyroid pill, within two hours of it, or six to eight hours after it.
What it can’t tell you
- We summarised it from the abstract only.
- This study followed people over time rather than randomly assigning them to groups from the start. It can show a link between timing and health changes but cannot prove one caused the other.
- These results apply to postmenopausal women taking thyroid medicine. They may not apply to other people taking calcium carbonate.
Who paid
- Authors work at
- University of Messina, Italy; University Hospital Policlinico G. Martino, Italy
The paper
- Title
- L-thyroxine malabsorption due to calcium carbonate impairs blood pressure, total cholesterolemia, and fasting glycemia
- Type
- Study
- Evidence
- Studies in people
- Summarised from
- Abstract only
- Cite
- Morini E, Catalano A, Lasco A, Morabito N, Benvenga S (2019). L-thyroxine malabsorption due to calcium carbonate impairs blood pressure, total cholesterolemia, and fasting glycemia. Endocrine. doi:10.1007/s12020-018-1798-7PubMed 30368654DOI
Summary written 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error