The purpose of this study is to help the researchers understand if using a soy supplement impacts cardiovascular disease risk factors in patients with subclinical hypothyroidism.

This pilot trial is a double blind, randomized, parallel arm trial. Eighty participants with subclinical hypothyroidism will be randomized to consume either soy protein isolate or milk protein isolate for 8 weeks.

Inclusion Criteria:

• Patients with subclinical hypothyroidism (otherwise healthy volunteers)
• Adults aged 25 - 70 yrs.
• Willing to avoid consumption of soy/other isoflavone containing foods (i.e. some snack bars) during the study.
• Willing to avoid consumption of dietary (other than multivitamin) and herbal supplements during the study.

Exclusion Criteria:

• Taking drugs that interfere with thyroid function
• Planning pregnancy in the next 6 months
• Taking drugs that lower lipids, blood pressure, or sensitize insulin
• Regular consumption of soy products (>20 g/wk)
• Consumption of soyfoods within 90 days prior to enrollment.
• Known history of soy or milk allergy or intolerance.
• Taking antibiotics during the intervention
• Active viral infections such as Human immunodeficiency virus (HIV) positive or hepatitis