Primary hypothyroidism is a common medical disorder that usually presents secondary to autoimmune thyroiditis. Symptoms of hypothyroidism include fatigue, weight gain, cold intolerance, and constipation. Treatment of hypothyroidism with levothyroxine is effective, but starting doses vary. There are two strategies for initiating levothyroxine in healthy adults younger than 65 years: low-dose (50 mcg) and full-dose (1.6 to 1.7 mcg per kg). Traditionally, lower starting doses of 25 to 50 mcg per day are recommended for patients 65 years and older or who have ischemic heart disease. Low-dose initiation of thyroid supplementation usually takes four to six months before plasma thyrotropin and free thyroxine (FT₄) levels become normal. Roos and associates compared the full starting dose of levothyroxine with the low dose in individuals with primary hypothyroidism without known cardiac disease.

The study was a prospective, randomized, double-blind trial of patients with hypothyroidism who received care at one hospital during the three-year study. Patients diagnosed with primary autoimmune hypothyroidism for the first time were included in the study. The age range of patients in the study was 22 to 86 years, with a median of 46 years. Patients were excluded if they had a history of cardiac disease or were taking cardiac medication. Patients were randomly assigned to receive a starting levothyroxine dosage of 25 mcg per day (low dose) or 1.6 mcg per kg per day (high dose). Levothyroxine doses were adjusted by 25 mcg every four weeks until 24 weeks. Doses then were adjusted every 12 weeks with the goal of maintaining serum thyrotropin and FT₄ levels within normal reference ranges.

Clinical scores of hypothyroidism and a general quality of life survey were evaluated at each visit. Cardiac symptoms and events were assessed at every visit, and a bicycle ergometry was performed at baseline and repeated at weeks 12 and 24.

Twenty-five patients were randomly assigned to the low-dose group and 25 to the high-dose group. The two groups were similar in severity of hypothyroidism and age. At week four, 13 patients reached euthyroid levels in the high-dose group compared with one in the low-dose group. This trend persisted until week 24, when 21 in the high-dose group and 20 in the low-dose group met the criteria for being euthyroid. The rate of change in clinical scores and quality of life were similar between the two groups. Body weight, heart rate, and blood pressure were unchanged in both groups. There were no palpitations, angina pectoris, or other cardiac events documented in either group.

The authors conclude that starting healthy primary hypothyroid patients on a full dose of levothyroxine is safe and effective for those who do not have a history of cardiac disease. They add that this strategy may be more convenient and cost effective than the method of starting low and titrating slowly.