Oral Estradiol and Hypothyroidism: Why Levothyroxine Requirements Can Change

If you have hypothyroidism and are starting oral oestradiol, there is a well-established pharmacological interaction you need to know about. It does not affect everyone on HRT — but for people on levothyroxine, it is one of the more clinically important drug interactions in endocrinology.

The mechanism: oestrogen and thyroxine-binding globulin

Oral oestradiol, via its hepatic first-pass effect, increases the liver’s production of several binding proteins — including thyroxine-binding globulin (TBG), SHBG, and corticosteroid-binding globulin (CBG), as well as coagulation factors. TBG is the main transport protein for thyroid hormones in the blood.

This route-dependent effect has been studied directly in trans populations. A 2025 prospective study of transfeminine people starting GAHT found TBG rose by approximately 24% to 48% with oral oestradiol (depending on whether it was combined with cyproterone acetate or a GnRH analogue), while transdermal oestradiol produced no significant change in TBG. An earlier study in euthyroid trans women and trans men similarly found that oral, but not transdermal, oestradiol increased TBG.

When TBG rises, more thyroid hormone becomes protein-bound, temporarily reducing free hormone concentrations until thyroid output adapts. In people with a functioning thyroid, the gland increases its output to compensate and free T4 normalises. TSH remains stable and there are no clinical consequences.

In people with hypothyroidism — particularly those on a fixed dose of levothyroxine with little or no residual thyroid function — this compensation cannot occur. Free T4 falls, TSH rises, and the person becomes functionally under-replaced, often experiencing a return of hypothyroid symptoms despite taking the same dose they were previously stable on.

The evidence

This is one of the classic interactions in endocrinology, though the foundational evidence on the clinical consequence — how much the levothyroxine dose needs to change — comes from cisgender women, not trans women. A landmark 2001 NEJM study by Arafah, conducted in postmenopausal cisgender women, demonstrated rising TBG, falling free T4, rising TSH, and increased levothyroxine requirements in hypothyroid women receiving oestrogen therapy. Pregnancy provides the same physiological model in cisgender women: high oestrogen levels raise TBG substantially, and hypothyroid women require on average significantly higher levothyroxine doses to maintain euthyroidism during pregnancy. The underlying mechanism — oral oestrogen raising TBG via first-pass hepatic metabolism — is the same one confirmed directly in trans women above; what hasn’t been directly studied in trans women on levothyroxine is the exact magnitude of dose increase needed, which is why the cisgender evidence is extrapolated for that specific question.

Transdermal oestradiol bypasses hepatic first-pass metabolism and has much smaller effects on TBG, which may make it particularly attractive for people with hypothyroidism on levothyroxine.

What to do in practice

If you have hypothyroidism and are starting oral oestradiol — or switching from transdermal to oral — your levothyroxine dose may need to increase. TSH should be rechecked approximately 6–8 weeks after initiating or changing oral oestradiol. This timing reflects the slow equilibration of TSH, which is standard endocrine practice for any change in thyroid replacement.

The magnitude of dose adjustment is individual and should be guided by repeat testing rather than a fixed formula.

What about people without thyroid disease?

Routine thyroid screening is not recommended as part of standard feminising HRT monitoring. The Endocrine Society, WPATH, and UCSF guidelines do not include TSH as a standard HRT monitoring test, and there is no evidence that HRT causes clinically important thyroid dysfunction in euthyroid individuals.

In people without thyroid disease, thyroid testing should follow standard primary-care screening practices rather than being considered mandatory HRT monitoring. The oral oestradiol–TBG interaction is a hypothyroid-specific clinical concern, not a population-level HRT risk.

If TSH is abnormal

If TSH is abnormal or symptoms are discordant with the result, free T4 should be measured. Additional testing — including thyroid antibodies or free T3 — may occasionally be useful in selected cases, but free T3 is not routinely recommended for diagnosing hypothyroidism and should not be requested as a default. Most clinical decisions in thyroid management are guided by TSH and free T4.

Sources

1. Arafah BM. Increased Need for Thyroxine in Women with Hypothyroidism during Estrogen Therapy. New England Journal of Medicine. 2001;344(23):1743–1749.
2. Bisschop PH, Toorians AW, Endert E, Wiersinga WM, Gooren LJ, Fliers E. The Effects of Sex-Steroid Administration on the Pituitary-Thyroid Axis in Transsexuals. European Journal of Endocrinology. 2006;155(1):11–16.
3. Stangl TA, Wiepjes CM, Heijboer AC, den Heijer M. The Influence of Gender-Affirming Hormone Therapy on Serum Concentrations of Hormone-Binding Proteins. European Journal of Endocrinology. 2025;192(5):614–620.
4. Jonklaas J, Bianco AC, Bauer AJ, Burman KD, Cappola AR, Celi FS, Cooper DS, Kim BW, Peeters RP, Rosenthal MS, Sawka AM. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751.