Table 3.

Hazard Ratios and 95% Confidence Intervals for the Association of Thyroid Cancer With Reproductive and Hormonal Factors, US Radiologic Technologists Study, 1983–2006^a

	No. of Casesb	HRc	95% CI
No. of births
Nulliparous	59	1.00	Reference
1	39	0.95	0.63, 1.42
2	85	0.97	0.69, 1.36
≥3	50	0.87	0.59, 1.29
P-trend*		0.61
Age at menarche, years
<12	71	1.00	Reference
12–13	122	0.65	0.49, 0.88
14–15	35	0.63	0.42, 0.94
≥16	9	1.45	0.72, 2.92
P-trend*		0.14
Oral contraceptive use
Never user	44	1.00	Reference
Ever user	197	1.28	0.91, 1.82
<5 years of use	100	1.29	0.89, 1.87
≥5 years of use	90	1.24	0.84, 1.81
Menopausal status
Premenopausal	196	1.00	Reference
Natural menopause	18	0.69	0.37, 1.28
Hysterectomy, both ovaries removed	18	0.85	0.49, 1.45
Hysterectomy, still has ≥1 ovaries/unknown	4	0.65	0.21, 1.96
Other/unknown reason	6	1.49	0.64, 3.46
Menopausal hormone therapy used
Never user	13	1.00	Reference
Ever user	33	1.16	0.61, 2.24

Abbreviations: CI, confidence interval; HR, hazard ratio.

*P-trend based on ordinal score variable.

^aWomen only (n = 69,506).

^bNumber of cases may be inconsistent because of missing values.

^cAdjusted for birth year, smoking status, body mass index, number of personal radiographs to the head or neck, cumulative occupational radiation dose, and medical history of benign thyroid conditions.

^dAmong postmenopausal women only.