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BACKGROUND: Earlier onset of puberty is associated with an increased risk for adverse health outcomes, including breast cancer. The thyroid hormone (TH) axis, a negative feedback loop of thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3), is a key regulator of growth, development, and reproductive function. Epidemiologic studies rarely have evaluated the role of THs in pubertal onset.
METHODS: We used data from an ongoing study of 6–8-year-old female Kaiser Permanente members enrolled during 2005–2006 and followed since. Annual anthropometry including breast and pubic hair Tanner staging were conducted. Serum TSH, free T4 (fT4), total T4 (tT4), and free T3 were measured once during the first, second, or third year for 324 subjects. Linear regression coefficients and Cox proportional hazards ratios (HRs) were adjusted for race/ethnicity, and age, body mass index, annual family income, and caregiver education at time of blood draw.
RESULTS: Age at blood draw was associated with fT4 (4.7% decrease/year of age; 95% confidence interval [CI]: 1.9%–7.5%) and TSH (9.1% increase/year; 95% CI: 2.1%–16.1%). fT4 concentrations were lowest among white non-Hispanics, and highest among Asians (13.0% difference; 95% CI 5.3%–20.6%). Higher pre-pubertal fT4 (HR: 1.10/fT4 standard deviation [SD]; 95% CI: 1.02–1.18) and tT4 (HR: 1.17/tT4 SD; 95% CI: 1.05–1.30) were associated with earlier pubic hair development. No associations were identified between onset of breast development and THs.
CONCLUSIONS: Higher T4 concentrations were associated with earlier pubic hair development, after accounting for anthropometric and demographic factors. The TH axis should be considered as a potential pathway regulating pubertal timing. Understanding factors influencing TH concentrations and signaling can be useful in predicting pubertal onset.