Table 1.

Characteristics of included randomised controlled trials

Study Author (year)/ Country	N	Intervention levothyroxine	Control	Use of assisted reproduction technologies (% of participants)	Inclusion of subclinical hypothyroidism	TSH (Normal range; mIU/L)	Defintion of TPOAb positivity (IU/mL)	Mean GA of LT4 initiation (weeks)		Baseline TSH (mIU/L) (median, *mean)
								Intervention Arm (LT4)	Control arm	Intervention Arm (LT4)	Control Arm
Dhillon-Smith et al (2019)/ UK8	952	Initiated preconception	Placebo	Yes (45)	No	0.44–3.63	>99% concordance with UK NEQAS IIA	Preconception	Preconception	2.1	2
Nazarpour et al (2017)/ Iran9	131	Initiated 4–8 days following first prenatal visit	No treatment	Unclear	Yes	0.1–2.5	>50	11.4±4.2	11.4±4.2	3.7	3.2
Negro et al (2005)/ Italy11	72	Initiated 1 month before assisted reproduction technologies	No treatment	Yes (100)	Yes	0.27–3.2	>100	Preconception	Preconception	1.9*	1.7*
Negro et al (2006)/ Italy10	115	Initiated at first endocrinological visit (2-7d after first obstetrics visit)	Placebo	No	No	0.27–4.2	>100	10.4±3.1	10.3±3.1	1.6*	1.7*
Negro et al (2016)/ Italy12	393	Initiated in first trimester prior to 12 weeks	No treatment	No	No	0.5–2.5	>16	7.1±1.2	7.0±1.2	1.4*	1.4*
Wang et al (2017)/ China14	600	Initiated 2–4 weeks prior to controlled ovarian hyperstimulation	No treatment	Yes (100)	No	0.45–4.78	60	Preconception	Preconception	2.9	2.1

FT4, free thyroxine; GA, gestational age; LT4, levothyroxine; NEQAS IIA, National External Quality Assurance Service IIA; TPOAb, thyroid peroxidase antibody; TSH, thyroid stimulating hormone.