Decreased thyroid T4 indicates hypothyroidism, which may be either hypothyroidism or Hashimoto’s thyroiditis, and requires simultaneous checking of T3, T4 and TSH levels. If hypothyroidism is determined to be the cause, oral thyroxine is required for symptomatic treatment. Thyroxine levels should be reviewed regularly during the course of treatment and medication may have to be adjusted based on the results of the review. In the case of Hashimoto’s thyroiditis, an autoimmune disease that manifests as a mass in the thyroid gland and hypothyroidism, treatment requires oral thyroxine tablets, and surgical removal may be considered when the mass has symptoms of compression. After thyroid surgery, T4 may be reduced due to insufficient thyroxine supplementation, which requires an increase in the oral dose of thyroxine and regular review.