Also indexed as: Animal Levothyroxine/Liothyronine, Animal
Thyroid, Armour Thyroid, Cytomel, Desiccated Thyroid, Eltroxin, Euthroid, L-Tri-iodothyronine,
Levo-T, Levothroid, Levothyroxine, Levothyroxine (Synthetic), Levoxyl, Liothyronine,
Liothyronine (Synthetic), Liotrix, Proloid, Synthroid, Thyar, Thyroglobulin, Thyrolar,
Thyroxine, Triostat, Unithroid
About thyroid hormones
Thyroid medications are synthetic or animal-derived hormones used to treat people with hypothyroidism (low thyroid function), goiter,
and Hashimoto’s disease.
Helpful Products
Try these helpful products which may be beneficial if taken with this medicine
- Calcium
- Levothyroxine can deplete this nutrient from the body; calcium supplements (1,000 mg per day) might
theoretically help, but should be taken an hour before or two hours after the drug to avoid an
interaction
- Iron
- Taking iron may improve thyroid hormone
levels in people who are deficient; get your levels checked by a doctor and ask about the best
time to take it, as iron interferes with levothyroxine absorption
These recommendations are not comprehensive and are not intended to
replace the advice of your doctor or pharmacist. Continue reading the full article for more
information on interactions with vitamins, herbs, and foods.
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
Soy
|
|
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.
Soy
Ingestion of soy products simultaneously with thyroid hormones appears to reduce the
absorption of the hormones. To be safe, people taking thyroid medication 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.
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.13
Interactions with Foods and Other Compounds
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
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. Centanni M, Gargano L, Canettieri G, et al. Thyroxine in goiter,
Helicobacter pylori infection, and chronic gastritis. N Engl J Med
2006;354:1787–95.
13. Brinker F. Herb Contraindications and Drug Interactions.
Sandy, OR: Eclectic Institute, 1997, 21, 29–30.
14. Benvenga S, Bartolone L, Squadrito S, et al. Delayed intestinal
absorption of levothyroxine. Thyroid 1995;5:249–53.
15. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and
Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991,
132–3c.
16. 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.
17. Threlkeld DS, ed. Hormones, Thyroid Hormones. In Facts and
Comparisons Drug Information. St. Louis, MO: Facts and Comparisons, Jun 1991,
132–3c.