Kelp meal (KM) is a supplement made from the brown seaweed Ascophyllum nodosum known to bioaccumulate iodine (I) in addition to be the richest of phlorotannins (PT). Previous research revealed that up to 5% of KM in the diet DM decreased CH₄ emission in vitro and inhibited ruminal archaea in vivo with these responses likely modulated by PT. However, feeding ≥2% of KM is challenging because high intake of I may negatively affect thyroid health. We designed a study in which 2% of KM (400 g/d) was fed during 3 wk (high phase) followed by 0.5% of the diet DM (100 g/d) for another 3 wk (stepdown phase) and complete removal during the last 3 wk of the experiment (withdraw phase). Our goal was to investigate the effect of KM on yields of milk and milk components, CH₄ emission, and thyroid hormones. Twenty Jersey cows (102 ± 52 DIM) were assigned to treatments in a randomized complete block design. Cows were fed (DM basis) 60% alfalfa hay and 40% concentrate. Milk and blood samples were collected on wk 3, 6, and 9. The GreenFeed system was used to measure CH₄ emission throughout the study. Data were analyzed using the MIXED procedure of SAS with repeated measures over time. Iodine intake from KM averaged 240 mg/d (high phase) and 60 mg/d (stepdown phase). This resulted in I intake 34- (high phase) and 8.5-fold (stepdown phase) greater than required (i.e., 7.1 g/d). No treatment by wk interactions were observed for any variable tested. Likewise, treatments did not affect DMI (mean = 20.4 kg/d) and milk yield (mean = 24 kg/d). Concentrations and yields of milk fat and true protein, as well as CH₄ production were also unchanged. Specifically, CH₄ production averaged 389 g/d (high phase), 394 g/d (stepdown phase), and 380 g/d (withdraw phase). Treatments did not affect plasma concentrations of thyroid stimulating hormone (mean = 0.445 µIU/mL), free triiodothyronine (mean = 4.43 ng/mL), free thyroxine (mean = 0.74 ng/mL), and cortisol (mean = 31 ng/mL). Overall, dietary inclusion of up to 2% KM did not affect CH₄ production or thyroid health, suggesting that greater levels of KM may be required to depress methanogenesis and thyroid function.