Table 4.

Frequency of Maternal Thyroid Medication Use by Type of Thyroid Medication and Timing of Use among Controls (n = 5875) and Cases (n = 14,067), National Birth Defects Prevention Study, 1997–2004^a

	Controlsb				Cases
	Timing of use during pregnancyc				Timing of use during pregnancyc
	Early	Late	Unknown	Total	Early	Late	Unknown	Total
Any thyroid medications	100	2	1	103	289	5	1	295
Thyroxine	94	0	1	95	266	1	1	268
Antithyroid medication	5	2	0	7	20	3	0	23
Unknown thyroid medication	2	0	0	2	3	1	0	4

^aCase counts are for all birth defect categories included in the present analysis. Eight control and 21 case mothers reported thyroid disease, but no thyroid medication use during the period 3 months before conception through the end of pregnancy.

^bOne control mother reported taking both thyroxine and an antithyroid medication.

^cEarly use is defined as any use three months before conception through the end of the first trimester. Late use is defined as use only during pregnancy month four through delivery. Unknown is defined as insufficient information to determine timing of use.