Shop products for Thyroid Hormones 
Also indexed as: Animal Levothyroxine/Liothyronine, Animal Thyroid, Armour® Thyroid, Cytomel®, Desiccated Thyroid, Eltroxin®, Euthroid®, L-Tri-iodothyronine, Levo-T®, Levotec®, Levothroid®, Levothyroxine, Levothyroxine (Synthetic), Levoxyl®, Liothyronine, Liothyronine (Synthetic), Liotrix, Proloid®, Synthroid®, Tertroxin, Thyar®, Thyroglobulin, Thyrolar®, Thyroxine, Tri-iodothyronine, Triostat®, Unithroid®
Thyroid medications are synthetic or animal-derived hormones used to treat people with hypothyroidism (low thyroid function), goitre, and Hashimoto’s disease.
The information in this article pertains to thyroid hormones in general. The interactions reported here may not apply to all the Also Indexed As terms. Talk to your doctor or pharmacist if you are taking any of these drugs.
Summary of
Interactions with Vitamins, Herbs, and Foods
In some cases, an herb or supplement may appear in more than one category, which may seem
contradictory. For clarification, read the full article for details about the summarized
interactions.
 May be Beneficial: Depletion or
interference—The medication may deplete or interfere with the absorption or
function of the nutrient. Taking these nutrients may help replenish them. |
Calcium |
 Avoid: Reduced drug absorption/bioavailability—Avoid
these supplements when taking this medication since the supplement may decrease the absorption
and/or activity of the medication in the body. |
Calcium Soya |
| Avoid: Adverse interaction—Avoid these supplements
when taking this medication because taking them together may cause undesirable or dangerous
results. |
Bugleweed* Lemon balm* |
 Check: Other—Before taking any of these supplements
or eating any of these foods with your medication, read this article in full for details. |
Iron |
| Side effect reduction/prevention |
None known |
| Supportive interaction |
None known |
An asterisk (*) next to an item in the summary indicates that the interaction is supported only by weak, fragmentary, and/or contradictory scientific evidence.
Interactions with Dietary Supplements
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.
Soya
Ingestion of soya products simultaneously with thyroid hormones appears to reduce the
absorption of the hormones. To be safe, people taking thyroid medication should not consume
soya 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 soya-based formula without consulting a pediatrician or pediatric
endocrinologist.11
Interactions with Herbs
Bugleweed (Lycopus virginicus, Lycopus europaeus) and lemon balm (Melissa officinalis) may interfere with the action of thyroid hormones and should not be used during treatment with thyroid hormones.12
Interactions with Foods and Other Compounds
Food
Taking levothyroxine with food may decrease its absorption.13 Levothyroxine
absorption is increased when taken on an empty stomach.14 High-fibre diets have
been shown to decrease levothyroxine absorption.15 Thyroid hormones should be taken
an hour before eating, at the same time very day.16
References
1. Kung AWC, Pun KK. Bone mineral density in premenopausal women receiving long-term physiological doses of levothyroxine. JAMA 1991;265:2688–91.
2. Schneider DL, Barrett-Connor EL, Morton DJ. Thyroid hormone use and bone mineral density in elderly men. Arch Intern Med 1995;155:2005–7.
3. Franklyn JA, Betteridge J, Daykin J, et al. Long-term thyroxine treatment and bone mineral density. Lancet 1992;340:9–13.
4. Schneyer CR. Calcium carbonate and reduction of levothyroxine efficacy. JAMA 1998;279:750.
5. Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine. JAMA 2000;283:2822–5.
6. Beard JL, Borel MJ, Derr J. Impaired thermoregulation and thyroid function in iron-deficiency anemia. Am J Clin Nutr 1990;52:813–9.
7. Beard JL, Borel MJ, Derr J. Impaired thermoregulation and thyroid function in iron deficiency anemia. Am J Clin Nutr 1990;52:813–9.
8. Beard J, Borel M, Peterson FJ. Changes in iron status during weight loss with very-low-energy diets. Am J Clin Nutr 1997;66:104–10.
9. Campbell NR, Hasinoff BB. Iron supplements: A common cause of drug interactions. Brit J Clin Pharmacol 1991;31:251–5.
10. Campbell NR, Hasinoff BB, Stalts H, et al. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med 1992;117:1010–3.
11. Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: The influence of soy-based formulas. J Am Coll Nutr 1997;16:280–2.
12. Brinker F. Herb Contraindications and Drug Interactions. Sandy, OR: Eclectic Institute, 1997, 21, 29–30.
13. Benvenga S, Bartolone L, Squadrito S, et al. Delayed intestinal absorption of levothyroxine. Thyroid 1995;5:249–53.
14. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.
15. Liel Y, Harman-Boehm I, Shany S. Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients. J Clin Endocrinol Metab 1996;81:857–9.
16. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991, 132–3c.
