Calcium

Thyroid hormones have been reported to increase urinary loss of calcium.¹ However, recent research suggests that, under most circumstances, taking thyroid hormones may not be associated with reduced bone density.^{2 , 3} Calcium supplementation for people taking long-term thyroid medication has not yet been proven to be either helpful or necessary.

Simultaneous ingestion of some calcium formulations with levothyroxine has been reported to reduce the effectiveness of levothyroxine.⁴ For example, 1,200 mg per day of calcium as calcium carbonate, taken along with levothyroxine, significantly reduced absorption of the thyroid hormone.⁵ Levothyroxine activity will not be blocked if it is taken in the morning and calcium carbonate is taken after lunch and dinner. Separating these medications by at least four hours is recommended.

Iron

Iron deficiency has been reported to impair the body’s ability to make its own thyroid hormones,⁶ which could increase the need for thyroid medication. In a preliminary trial, iron supplementation given to iron-deficient women with low blood levels of thyroid hormones, partially normalized these levels.⁷ Diagnosing iron deficiency requires the help of a doctor. The body’s ability to make its own thyroid hormones is also reduced during low-calorie dieting. Iron supplementation (27 mg per day) was reported in a controlled study to help maintain normal thyroid hormone levels in obese patients despite a very low-calorie diet.⁸

However, iron supplements may decrease absorption of thyroid hormone medications.^{9 , 10} People taking thyroid hormone medications should talk with their doctor before taking iron-containing products. If advised to supplement, iron and the drug should not be taken within less than four hours of each other.

Soy

Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.¹¹

Betaine hydrocholoride

The normal stomach produces hydrochloric acid, which influences the absorption of thyroid hormones. Research has shown that taking drugs that inhibit the production of stomach acid reduces the absorption of thyroid hormones.¹² Conversely, people with low stomach acid who take betaine hydrochloride for "acid-replacement therapy" would be expected to have an increase in their absorption of thyroid hormones. For that reason, people taking thyroid hormones should not take betaine hydrochloride without the supervision of a doctor, who can determine whether a change in thyroid hormone dose is necessary.

Bugleweed (Lycopus virginicus, Lycopus europaeus) and lemon balm(Melissa officinalis) may interfere with the action of thyroid hormones. Check with your doctor before taking these herbs with thyroid hormones.¹³

Food

Taking levothyroxine with food may decrease its absorption.¹⁴ Levothyroxine absorption is increased when taken on an empty stomach.¹⁵ High-fiber diets have been shown to decrease levothyroxine absorption.¹⁶ Thyroid hormones should be taken an hour before eating, at the same time very day.¹⁷