Calcium carbonate, thyroid medicine absorption and heart risk markers
What it found
In postmenopausal women taking L-thyroxine, taking calcium carbonate within 2 hours of the tablet raised TSH and was linked to higher cholesterol, blood sugar and blood pressure.
Moving calcium to 6 to 8 hours later brought these back down.
What they found
Studies in people
Thyroid hormone levels
TSH was higher when calcium was taken within 2 hours of L-thyroxine (3.33 mU/L) than when it was taken 45 to 60 minutes before breakfast (1.93 mU/L) or 6 to 8 hours later (2.16 mU/L).
Elevated TSH prevalence
In the setting where calcium was taken within 2 hours, 18% of women had elevated TSH above 4.12 mU/L. No women had elevated TSH in the other two settings.
Cholesterol, blood sugar, blood pressure
Total cholesterol, fasting blood sugar, and systolic and diastolic blood pressure were all higher when calcium was taken within 2 hours of L-thyroxine compared with the other two settings.
TSH and heart risk markers
For every 1.0 mU/L rise in TSH between 0.85 and 6.9 mU/L, total cholesterol rose by 12.1 mg/dL, fasting blood sugar by 3.12 mg/dL, systolic blood pressure by 2.31 mmHg and diastolic blood pressure by 2.0 mmHg.
What the authors conclude
“Monitoring of hypothyroid patients who ingest medications that decrease L-thyroxine absorption should not be restricted to solely measuring serum TSH.”
Also in their conclusions
- They advise that monitoring of hypothyroid patients who take medicines that decrease L-thyroxine absorption should not be restricted to only measuring serum TSH.
How it was done
Calcium carbonate was already known to interfere with L-thyroxine absorption. The authors wanted to measure how much this malabsorption raised TSH and what that meant for cholesterol, blood sugar and blood pressure.
They followed postmenopausal hypothyroid women on L-thyroxine who added calcium carbonate 600 to 1000 mg a day. The women took L-thyroxine 45 to 60 minutes before breakfast (setting 1), then took calcium within 2 hours after L-thyroxine for 2.3 years (setting 2), and then postponed calcium 6 to 8 hours after L-thyroxine (setting 3). The authors measured TSH, total cholesterol, fasting blood sugar and blood pressure in each setting.
What it can’t tell you
- We summarised it from the abstract only.
- This was a cohort study, so it can show links between calcium timing, TSH and heart risk markers, but it cannot prove that the timing change alone caused the differences.
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 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error