Progesterone increases thyroid hormone
Progesterone production is often diminished by inflammation or stress that divert the common precursor to the cortisol pathway, and suboptimal thyroid function is widespread. A study just published in Clinical Endocrinology provides evidence that progesterone therapy increases thyroid hormone production. The authors state:
"Thyroid hormones and progesterone both influence core temperature, metabolism and are crucial during pregnancy. Our objective was to discover whether progesterone therapy caused changes in thyroid physiology compared with placebo."
To find out, they subjected their study subjects, women for whom it had been one to eleven years since their last menstrual period, had not been on either thyroid or ovarian hormone therapy, and were experiencing vasomotor symptoms (VMS = hot flashes), to treatment with oral micronized progesterone (with a randomized placebo cohort). They were looking for changes in TSH, free T3 and free T4 on progesterone compared to placebo. They confirmed a marked effect on the thyroid production of T4 (thyroxine):
"Women with thyroid data (69 of 133 in original trial) were randomized to progesterone (n = 39) or placebo (n = 30)—baseline thyroid values were normal. There were no VMS-thyroid interactions—VMS Score (number × intensity) did not correlate with TSH, FreeT3 or FreeT4. At 12 weeks on progesterone, TSH levels tended to be lower (1.7 mU) than on placebo (2.2); FreeT4 levels were higher (16.4 pmol/l) than on placebo (15.3). FreeT3 was unchanged throughout. Analysis of covariance showed a significant increase in FreeT4 on progesterone (+2.5 pmol/l; 1.9–3.0) vs on placebo (+1.7; 1.1–2.4)..."
Since free T3 is where the "rubber meets the road" regarding the main thyroid effect on tissues and that impaired conversion of T4 to T3 is common, this is a dimension of the study that deserves further investigation. Nonetheless, the starting point is T4, and the authors conclude:
"These are the first randomized controlled trial data to show that treatment with luteal phase equivalent doses of oral micronized progesterone is associated with a significant increase in Free T4 values. This 12-week randomized controlled progesterone trial documented that Free T4 levels were significantly elevated in progesterone-treated compared with placebo-treated women."