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. 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.

Thyroid hormones have been reported to increase urinary loss of calcium. However, recent research suggests that, under most circumstances, taking thyroid hormones may not be associated with reduced bone density. 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. For example, 1,200 mg per day of calcium as calcium carbonate, taken along with levothyroxine, significantly reduced absorption of the thyroid hormone. Calcium citrate and calcium acetate have also been shown to decrease levothyroxine absorption. Levothyroxine absorption will not be blocked if it is taken in the morning and calcium is taken after lunch and dinner. Separating these medications by at least four hours is recommended.

Iron deficiency has been reported to impair the body’s ability to make its own thyroid hormones, 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. 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.

However, iron supplements may decrease absorption of thyroid hormone medications. 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.