Drug products: Eltroxin, Euthyrox, Levaxin 0,1 mg u/lactose, Levaxin utan lactos, Levaxin®, Levothyroxin, Levothyroxine Sodium, Levothyroxine Sodium App, Levotyroxin Nycomed, L-Thyrox HEXAL, L-Thyroxin, L-Thyroxin AL, L-Thyroxin Aventis, L-Thyroxin Henning, L-Thyroxin Henning® Inject, L-Thyroxine Christiaens, SYNTHROID, Tirosint
ATC code: H03AA01
Substances: levothyroxine, levothyroxine sodium, levothyroxine sodium (hydrate)
The main indication of levothyroxine is hypothyreoidism.
Dosing of levothyroxine is individual and the lowest effective dose should be used. There are studies indicating that women required higher dose than men when measured as mg/kg of ideal weight. During estrogen therapy or pregnancy, increase of dose is usually needed because elevated thyroxine-binding globulin lead to decrease in free thyroxine serum concentration.
Some studies indicate that long term use of levothyroxine may lead to reduction of bone mineral density (BMD) in women, especially in premenopausal women. No changes in BMD in men have been recorded.
No studies with a clinically relevant sex analysis regarding the pharmacokinetics of levothyroxine have been found.A retrospective observational study (69 men, 88 premenopausal women, 91 postmenopausal women) showed that premenopausal women had a greater dose requirement (mg/kg bodyweight) of levothyroxine than men and postmenopausal women . However, a prospective observational study (13 men, 37 women) showed that when ideal body weight instead of actual body weight was used to calculate the dose, all women had a greater dose requirement than men .
In women on oral estrogen therapy or pregnant women, adaptation of dosage is necessary because of elevated thyroxine-binding globulin leading to decreased free thyroxine serum concentration . Levothyroxine-treated pregnant women often need to increase the daily dose by 25-30%. Following delivery, levothyroxine dose should be reduced to prepregnancy levels .
See Pharmacokinetics and dosing. No other studies with a clinically relevant sex analysis regarding the effects of levothyroxine have been found.
A review (in total 3279 patients) described that most cross-sectional studies reported adverse effects at a larger number of bone sites in postmenopausal women, suggesting a negative influence on bone mineral density (BMD) with levothyroxine therapy. However, other cross-sectional studies found comparable results in pre- and postmenopausal women, but no effect of levothyroxine on BMD in men. This review reports no dose-response relationship . The original manufacturer report that long-term levothyroxine therapy might induce decreased BMD in women, particularly postmenopausal women on higher doses .
Estrogen can increase the concentration of thyroxine-binding globulin in serum. Women using contraceptives containing estrogen or hormone replacement therapy may require higher doses of levothyroxine . Regarding drug-drug interactions aspects, please consult Janusmed Interactions (in Swedish, Janusmed interaktioner).
Regarding teratogenic aspects, please consult Janusmed Drugs and Birth Defects (in Swedish, Janusmed fosterpåverkan).
A cross-sectional retrospective study (33238 men, 87165 women) examined the relationship between TSH and free T4. In individuals not receiving levothyroxine treatment, free T4 concentrations corresponded to higher median TSH in men than in women .
Date of litterature search: 2014-09-12
Reviewed by: Mia von Euler
Approved by: Karin Schenck-Gustafsson