Synthroid Sexual Function Impact: What Levothyroxine Does to Libido, Arousal, and Performance

Does levothyroxine improve libido and sexual function?

The effect is indirect. Levothyroxine replaces thyroid hormone, and its labeled indications are hypothyroidism and, as an adjunct to surgery and radioiodine, TSH suppression in thyroid cancer [4]. Studies in people with thyroid disorders found that several sexual-function measures improved after thyroid hormone levels returned to normal, although improvement was incomplete [1][2].

The clearest prospective data come from a multicenter study of 48 men, 34 with hyperthyroidism and 14 with hypothyroidism, assessed when they presented and again 8 to 16 weeks after recovery from the thyroid disorder [1]. The authors concluded that most patients with thyroid hormone disorders have some sexual dysfunction and that it can be reversed by normalizing thyroid hormone levels [1].

Reversal is real but uneven. Ejaculation outcomes improved in the treated men, while in women the domains that normalized were desire, satisfaction and pain, with arousal and orgasm left unchanged [1][2].

What the evidence shows in men

In the 14 hypothyroid men, the prevalence of hypoactive sexual desire, delayed ejaculation and erectile dysfunction was 64.3%, and of premature ejaculation 7.1% [1]. Delayed ejaculation improved in half of them after treatment, and ejaculation latency time fell from a mean of 21.8 to 7.4 minutes (P<0.01) [1]. Scores on the International Index of Erectile Function changed significantly in several subdomains, and changes in circulating sex steroids partly reflected changes in sex hormone-binding globulin [1].

The hormonal background fits. A review of thyroid dysfunction and hypogonadism in men reports that primary hypothyroidism is associated with hypogonadotropic hypogonadism that is reversible with thyroid hormone replacement, and that free testosterone concentrations are reduced in men with primary hypothyroidism and normalize with replacement [5]. Prolactin is not consistently elevated in these men except in longstanding severe hypothyroidism, but it declines with replacement [5]. A separate review of thyroid function and reproduction notes that erectile abnormalities have been reported and that sperm morphology abnormalities in hypothyroidism normalize when euthyroidism is reached [6].

What the evidence shows in women

Two studies measured sexual function with the Female Sexual Function Index (FSFI).

The first compared 25 women with overt hypothyroidism, 11 with subclinical hypothyroidism and TSH above 10 mU/L, 14 with subclinical hypothyroidism at or below 10 mU/L, and 20 healthy controls [3]. Female sexual dysfunction was diagnosed in 56%, 54.6%, 14.6% and 15% respectively (P=0.006) [3]. Mean total FSFI scores were 23.9, 26.03, 29.2 and 32.30 (P<0.0001) [3]. Hyperprolactinemia, hyperlipidemia and depression were associated with sexual dysfunction in the overt hypothyroidism group [3].

The second followed 56 women with thyroid disease (22 hyperthyroid, 17 hypothyroid and 17 with euthyroid Hashimoto thyroiditis) plus 30 controls, scoring them at diagnosis and again after normal thyroid function was restored [2]. All sexual domain scores were reduced in both hypothyroid and hyperthyroid women [2]. After correction of hypothyroidism, desire, satisfaction and sexual pain normalized, while arousal and orgasm remained unchanged, and the authors note that a longer period may be needed for full normalization [2]. Euthyroid women with Hashimoto thyroiditis also had reduced desire without changes in other domains; the authors say preliminary data suggest thyroid autoimmunity may impair desire independently of thyroid function [2].

Menstrual change is the other common reproductive complaint: hypothyroidism is associated mainly with oligomenorrhea [6], and the levothyroxine label lists menstrual irregularities among the reproductive adverse reactions of overdosage [4].

How is levothyroxine dosed and monitored?

The label's main safety concern with replacement is over- or undertreatment. Levothyroxine has a narrow therapeutic index, and both states may negatively affect reproductive function along with cardiovascular function, bone metabolism, growth, cognition, gastrointestinal function and glucose and lipid metabolism [4].

Per the levothyroxine sodium tablet label, a full replacement dose in adults is 1.6 mcg per kg per day, titrated in 12.5 to 25 mcg steps every 4 to 6 weeks until the patient is euthyroid and TSH has returned to normal, and the peak effect of a given dose may take 4 to 6 weeks [4]. In secondary or tertiary hypothyroidism, the label says TSH is not a reliable measure and free T4 is used instead [4].

The AACE/ATA 2012 guidelines call serum TSH the single best screening test for primary thyroid dysfunction, name L-thyroxine as standard treatment, and say the decision to treat subclinical hypothyroidism at a TSH below 10 mIU/L should be tailored to the individual patient [7].

Can levothyroxine cause sexual side effects?

The label lists no libido, arousal or erectile adverse reaction. The adverse reactions listed for levothyroxine are primarily those of hyperthyroidism from overdosage, and the reproductive entries are menstrual irregularities and impaired fertility [4]. The rest of that list is the familiar hyperthyroid picture: palpitations, tachycardia, arrhythmias, anxiety, nervousness, insomnia, tremor, weight loss, heat intolerance, sweating and diarrhea [4].

Thyroid hormone excess has its own sexual pattern in the studies. In hyperthyroid men, premature ejaculation prevalence was 50% and fell to 15% after treatment, while ejaculation latency doubled from 2.4 to 4.0 minutes [1]. In hyperthyroid women, treatment normalized desire, arousal and lubrication, satisfaction and pain, but orgasm remained impaired [2].

What can leave thyroid levels low despite treatment?

Drug interactions and gastrointestinal conditions can reduce levothyroxine absorption and may leave thyroid hormone levels inadequately controlled. Calcium carbonate, ferrous sulfate, omeprazole, H. pylori infection and atrophic gastritis were associated with reduced treatment effect or higher thyroxine requirements in the cited studies [8][9][10].

Calcium carbonate providing 1,200 mg of elemental calcium daily, taken with levothyroxine for 3 months by 20 patients, lowered mean free T4 from 17 to 15 pmol/L and raised TSH from 1.6 to 2.7 mIU/L; TSH was 1.4 mIU/L after calcium stopped, and 20% of patients had TSH above the normal range [8]. Ferrous sulfate 300 mg daily, taken with thyroxine for 12 weeks by 14 patients, raised mean TSH from 1.6 to 5.4 mU/L, and 9 patients had more symptoms of hypothyroidism [9]. In 10 patients on thyroxine, omeprazole raised TSH in all of them, an effect reversed by a 37% dose increase, and thyroxine requirements were 22% to 34% higher in patients with H. pylori gastritis or atrophic gastritis [10].

The levothyroxine sodium tablet label says to take it once daily on an empty stomach one-half to one hour before breakfast, at least 4 hours apart from phosphate binders such as calcium carbonate and ferrous sulfate, and at least 4 hours before bile acid sequestrants or monitor TSH. It says to monitor when proton pump inhibitors, antacids or sucralfate are used [4]. The label also advises stopping biotin and biotin-containing supplements at least 2 days before TSH or T4 testing [4].

Limits of the evidence

The studies above describe what happens to sexual function when thyroid disease is treated, and they are small (14 hypothyroid men and 17 hypothyroid women in the two treatment-outcome studies) [1][2]. They do not show how to dose levothyroxine for a sexual-function goal.