Oral Contraceptive Pills and Hyperthyroidism
Oral contraceptive pills (OCPs) do not cause hyperthyroidism, though they can affect thyroid hormone measurements without causing clinical thyroid dysfunction. 1
Effects of OCPs on Thyroid Function
Physiological Changes
- OCPs, particularly those containing estrogen, increase hepatic synthesis and release of thyroxine binding globulin (TBG) 1, 2
- This increase in TBG results in elevated total thyroxine (T4) and triiodothyronine (T3) levels by 20-40%, but free thyroid hormone levels typically remain within normal range 3
- Despite these laboratory changes, patients remain clinically euthyroid as the free hormone levels (free T3 and free T4) are minimally affected 1, 3
Different OCP Formulations
- Dienogest-containing OCPs cause no change in free T4 and only transitory reduction in free T3 during the first cycle 3
- Levonorgestrel-containing OCPs may cause slight increases in free T4 while free T3 remains unchanged 3
- The magnitude of effect on thyroid binding proteins appears to be related to the ethinyl estradiol dose in the OCP 3
Clinical Implications
Laboratory Assessment
- When evaluating thyroid function in women taking OCPs, the calculated free T4 level is the critical laboratory parameter rather than total T4 1
- If free T4 results are equivocal, additional testing such as thyroid-stimulating hormone (TSH) levels or thyrotropin-releasing hormone tests may be necessary 1
- It is unnecessary to discontinue OCPs to accurately evaluate thyroid function 1
Risk Considerations
- There is no endocrine contraindication to using OCPs in women with a history of hyperthyroidism who are in remission 1
- Women with subclinical hypothyroidism taking OCPs may have increased cardiovascular and thromboembolic risk due to the estrogen component 2
- Modern monophasic OCPs typically contain ≤50 μg of ethinyl estradiol combined with newer generation progestins, which were developed to reduce cardiovascular risks 4
Monitoring Recommendations
Baseline Assessment
- Blood pressure should be assessed before initiating any OCP, as estrogen-containing contraceptives can increase blood pressure in some users 4
- Women with a history of thyroid disease should have baseline thyroid function tests including TSH and free T4 1, 5
Follow-up
- Routine monitoring of thyroid function may be warranted in women with a history of thyroid disease who start OCPs 5
- When interpreting thyroid tests in women taking OCPs, clinicians should be aware of the expected increase in total T3 and T4 without clinical hyperthyroidism 1, 3
Special Considerations
Newer OCP Formulations
- Newer OCPs containing natural estrogens (estradiol valerate, estetrol) combined with newer progestins may have less impact on hepatic protein production 6
- Drospirenone-containing OCPs have shown promise in mitigating adverse effects on blood pressure through anti-mineralocorticoid effects 6