Hypothyroidism (underactive thyroid gland) is a common condition in women around menopause (40 years and above). Incidentally this group is also at a risk of developing osteoporosis (bone loss); hence such individuals may be recommended both thyroid hormone replacement drugs (such as levothyroxine) and calcium supplements.
Studies have found that taking calcium supplements and thyroid hormone replacement drugs (such as levothyroxine) at the same time can interfere with the absorption of thyroxine. Hence it is advisable to take the two 6-12 hours apart.
To substantiate the same, a study was conducted involving 20 female patients on hypothyroid treatment. The thyroxine level in blood was measured before starting the calcium supplementation and then during the three-month period when the patients were on calcium supplementation (patients were told to take the calcium supplement at the same time as the thyroid medication). The thyroxine levels were also measured several months after stopping calcium supplementation. It was observed that there was a moderate but a significant effect on thyroid function during the period patients took calcium. The blood tests of 4 patients showed that the thyroid medicine was not entering the blood.
In fact, some earlier studies have also shown that other widely used therapies, such as aluminum hydroxide (constituent of many antacids), iron supplements and sucralfate commonly prescribed for stomach and intestinal disorders also hinder the absorption of thyroxine.
Hence, it is essential for the thyroid patients to tell their doctor about any other medications they are taking especially, over-the-counter medications such as calcium supplements, iron supplements etc. Taking these supplements at the same time as the thyroid medication can worsen the condition in hypothyroid patients.