Hormonal birth control (HBC) and the thyroid gland are central to the endocrine system, which regulates metabolism and overall bodily function. The thyroid gland produces thyroxine (T4) and triiodothyronine (T3), which travel through the bloodstream. The synthetic hormones, particularly estrogen, found in most oral contraceptives, patches, or rings, interact directly with this thyroid hormone pathway. This interaction alters the body’s natural hormone balance.
How Hormonal Birth Control Changes Thyroid Hormone Levels
The interaction centers on Thyroid-Binding Globulin (TBG), a protein produced by the liver. Estrogen, a component in most combined hormonal contraceptives, stimulates the liver to produce more TBG. TBG binds to T4 and T3, acting as a transportation vehicle for these hormones in the blood.
When TBG concentration increases due to hormonal birth control, more circulating thyroid hormone becomes bound and inactive. This binding elevates the measure of total T4 and T3 levels in a blood test. However, only the free, or unbound, hormone is biologically active and regulates metabolism.
The increase in bound hormone reduces the available free T4 and T3. The net effect of the estrogen component is a shift in thyroid hormone distribution, lowering the concentration of the active, free forms the body relies on.
Impact on Individuals with Healthy Thyroid Function
Individuals with a healthy thyroid gland can typically compensate for the increased binding capacity caused by hormonal birth control. The body’s regulatory system, the hypothalamic-pituitary-thyroid axis, maintains a stable level of active, free hormone by detecting the slight drop in free T4 and T3.
In response to low free hormone levels, the pituitary gland releases more Thyroid-Stimulating Hormone (TSH). TSH signals the thyroid gland to increase its production of T4 and T3. The healthy thyroid responds by making more hormone, overcoming the increased binding by TBG.
This compensatory mechanism usually restores the balance of free T4 and T3 to normal. Therefore, hormonal birth control generally does not cause clinically significant hypothyroidism or hyperthyroidism in individuals without an underlying thyroid condition. While blood tests might show elevated total T4, the TSH and free T4 levels often remain within the normal range.
Monitoring and Adjusting Thyroid Replacement Medication
The interaction is particularly important for individuals with hypothyroidism taking synthetic thyroid replacement medication, such as levothyroxine (synthetic T4). Unlike a healthy thyroid, which increases hormone output when stimulated by TSH, a person on levothyroxine cannot naturally increase production.
When someone on a stable levothyroxine dose starts an estrogen-containing contraceptive, the estrogen increases TBG production. This increased TBG binds to the administered levothyroxine, resulting in less free, active hormone available. This can cause a functional hypothyroid state, potentially leading to symptoms like fatigue or weight gain.
Close monitoring is necessary when starting or stopping hormonal birth control while taking levothyroxine. A doctor typically recommends TSH testing six to eight weeks after starting or stopping the contraceptive. The levothyroxine dosage often needs to be increased to satisfy the increased TBG binding and leave an adequate amount of free T4.
Conversely, if a person stops taking estrogen-containing birth control, TBG levels gradually decrease. The existing levothyroxine dose may then be too high because less of it is being bound. This shift can potentially lead to symptoms of hyperthyroidism. Patients should inform their physician immediately when making changes to contraception use so that appropriate adjustments to the levothyroxine dose can be made.