How Levothyroxine (Synthroid) Affects TPO Antibodies

Levothyroxine restores hormone; it does not target antibodies

Levothyroxine sodium is synthetic T4, taken once daily, and the tablet prescribing information labels it as replacement therapy for primary, secondary and tertiary hypothyroidism [1]. TPO antibodies are autoantibodies against thyroid peroxidase, the enzyme the thyroid uses to build hormone, and they are a common marker of autoimmune thyroid disease such as Hashimoto's thyroiditis.

The label's treatment goal is to titrate until the patient is clinically euthyroid and serum TSH returns to normal [1]. Antibody levels are not part of that goal.

What the label says to follow instead

In adults with primary hypothyroidism, the label gives a full replacement dose of 1.6 mcg/kg/day, titrated in 12.5 to 25 mcg steps every 4 to 6 weeks until the patient is euthyroid [1]. TSH is the standard check in primary hypothyroidism. In secondary or tertiary hypothyroidism the label says TSH is not a reliable measure and free T4 should be used instead [1]. The label also advises stopping biotin and biotin-containing supplements at least 2 days before TSH and/or T4 testing [1].

Selenium is the intervention with TPO antibody data

A 2010 systematic review of prospective studies in people with Hashimoto's thyroiditis already taking levothyroxine included a meta-analysis of randomized, placebo-controlled, blinded trials of selenium [2]. After 3 months, TPO antibody titers were significantly lower with selenium than with controls, with a random-effects weighted mean difference of −271.09 (four studies; 95% CI −421.98 to −120.19; P < 10⁻⁴) [2]. Three studies also reported a higher chance of improved well-being or mood (risk ratio 2.79, 95% CI 1.21 to 6.47) [2].

The review found levothyroxine requirements and thyroid ultrasound morphology either unchanged or underreported, and its authors said improvement in thyroid function and morphology should be demonstrated before routine selenium supplementation is recommended [2].