ObjectiveMinor variation in serum thyroid hormone (TH) levels can have important effects on various clinical endpoints. Although 45–65% of the inter-individual variation in serum TH levels is due to genetic factors, the causative genes are not well established. We therefore studied the effects of genetic variation in 68 TH pathway genes on serum TSH and free thyroxine (FT₄) levels.Design and methodsSixty-eight genes (1512 polymorphisms) were studied in relation to serum TSH and FT₄ levels in 1121 Caucasian subjects. Promising hits (P<0.01) were studied in three independent Caucasian populations (2656 subjects) for confirmation. A meta-analysis of all four studies was performed.ResultsFor TSH, eight PDE8B polymorphisms (P=4×10⁻¹⁷) remained significant in the meta-analysis. For FT₄, two DIO1 (P=8×10⁻¹²) and one FOXE1 (P=0.0003) polymorphisms remained significant in the meta-analysis. Suggestive associations were detected for one FOXE1 (P=0.0028) and three THRB (P=0.0045) polymorphisms with TSH, and one SLC16A10 polymorphism (P=0.0110) with FT₄, but failed to reach the significant multiple-testing corrected P value (P<0.0022 and P<0.0033 respectively).ConclusionsUsing a large-scale association analysis, we replicated previously reported associations with genetic variation in PDE8B, THRB, and DIO1. We demonstrate effects of genetic variation in FOXE1 on serum FT₄ levels, and borderline significant effects on serum TSH levels. A suggestive association of genetic variation in SLC16A10 with serum FT₄ levels was found. These data provide insight into the molecular basis of inter-individual variation in TH serum levels.