Does hypothyroidism (underactive thyroid) affect male fertility?

Does Hypothyroidism Affect Male Fertility?

Yes, hypothyroidism negatively impacts male fertility and should be treated promptly, as the effects on spermatogenesis are reversible with levothyroxine replacement therapy. 1, 2

Direct Effects on Sperm Parameters

Hypothyroidism causes specific, measurable impairments in spermatogenesis:

• Sperm morphology is the most significantly affected parameter, with hypothyroid men showing substantially worse morphology compared to controls (p < 0.0001), and this improves significantly after 6-9 months of levothyroxine treatment (p < 0.001). 2

• Sperm motility may also be reduced in hypothyroid men, though the evidence is less robust than for morphology, and motility tends to improve with thyroid hormone replacement. 2

• The teratozoospermia index (TZI) correlates directly with free thyroxine levels, meaning lower thyroid hormone levels are associated with more abnormal sperm forms. 2

• Hypothyroidism decreases sex hormone-binding globulin (SHBG) levels, which can alter the bioavailability of testosterone and affect reproductive function. 3, 4

Hormonal Mechanisms

The reproductive dysfunction in hypothyroidism operates through multiple pathways:

• Primary hypothyroidism causes hypogonadotropic hypogonadism, characterized by subnormal LH responses to GnRH stimulation and reduced free testosterone concentrations, both of which normalize with thyroid hormone replacement. 4

• In severe, longstanding primary hypothyroidism, prolactin may be elevated (particularly in children and men with severe disease), which can further suppress gonadotropin secretion and impair fertility. 4

• Hypothyroidism reduces SHBG concentrations, altering the balance of bound and free sex steroids. 3, 4

Clinical Management Algorithm

Step 1: Diagnose and Confirm Hypothyroidism

• Measure morning TSH and free T4 levels to confirm hypothyroidism. 3
• Avoid testing during acute illness, as this can produce misleading results. 3

Step 2: Obtain Baseline Fertility Assessment

• Perform semen analysis before initiating levothyroxine to document baseline parameters. 3, 2
• Measure FSH, LH, testosterone, and prolactin to characterize the degree of hypogonadotropic hypogonadism. 3, 4

Step 3: Initiate Levothyroxine Replacement

• Start levothyroxine at an appropriate dose based on the severity of hypothyroidism and patient characteristics. 1
• Critically important: Levothyroxine undertreatment or overtreatment can negatively affect reproductive function, so careful dose titration is essential. 1
• Monitor TSH and free T4 at 6-8 weeks after initiation and after any dose change. 1

Step 4: Reassess Fertility After Achieving Euthyroidism

• Repeat semen analysis 6-9 months after achieving stable euthyroid status, as this is the timeframe in which sperm parameters improve. 2
• Spermatogenesis takes approximately 74 days (2.5 months), so full effects require at least this duration. 5

Step 5: Consider Additional Interventions if Needed

• If semen parameters remain abnormal despite euthyroidism, refer to a male reproductive specialist for evaluation of other causes. 3
• Assisted reproductive technology (IVF/ICSI) may be necessary if severe oligospermia or other abnormalities persist. 3

Critical Pitfalls to Avoid

Never prescribe exogenous testosterone to hypothyroid men seeking fertility, as testosterone therapy suppresses FSH and LH through negative feedback on the hypothalamus and pituitary, causing further impairment or complete suppression of spermatogenesis that can take months to years to recover. 3, 5

Do not conclude that fertility is permanently impaired before achieving stable euthyroidism for at least 6-9 months, as the reproductive abnormalities are reversible with appropriate thyroid hormone replacement. 2

Screen all men presenting with abnormal semen parameters for thyroid dysfunction before initiating other fertility treatments, as correcting hypothyroidism may resolve the fertility issue without need for assisted reproductive technology. 2

Reversibility and Prognosis

The reproductive effects of hypothyroidism are fully reversible with appropriate levothyroxine replacement therapy:

• Sperm morphology normalizes or significantly improves after 6-9 months of treatment. 2
• Free testosterone concentrations normalize with thyroid hormone replacement. 4
• Hypogonadotropic hypogonadism resolves, with LH responses to GnRH returning to normal. 4

The longer hypothyroidism persists untreated, the greater the potential damage to the testes, making early diagnosis and treatment essential. 6