The Swinging Pendulum in Treatment for Hypothyroidism: From (and Toward?) Combination Therapy

Abstract

Thyroid hormone replacement for hypothyroidism can be achieved via several approaches utilizing different preparations of thyroid hormones, T3, and/or T4. "Combination therapy" involves administration of both T3 and T4, and was technically the first treatment for hypothyroidism. It was lauded as a cure for the morbidity and mortality associated with myxedema, the most severe presentation of overt hypothyroidism. In the late nineteenth and the early Twentieth centuries, combination therapy per se could consist of thyroid gland transplant, or more commonly, consumption of desiccated animal thyroid, thyroid extract, or thyroglobulin. Combination therapy remained the mainstay of therapy for decades despite development of synthetic formulations of T4 and T3, because it was efficacious and cost effective. However, concerns emerged about the consistency and potency of desiccated thyroid hormone after cases were reported detailing either continued hypothyroidism or iatrogenic thyrotoxicosis. Development of the TSH radioimmunoassay and discovery of conversion of T4-to-T3 in humans led to a major transition in clinical practices away from combination therapy, to adoption of levothyroxine "monotherapy" as the standard of care. Levothyroxine monotherapy has a favorable safety profile and can effectively normalize the serum TSH, the most sensitive marker of hypothyroidism. Whether levothyroxine monotherapy restores thyroid hormone signaling within all tissues remains controversial. Evidence of persistent signs and symptoms of hypothyroidism during levothyroxine monotherapy at doses that normalize serum TSH is mounting. Hence, in the last decade there has been acknowledgment by all thyroid professional societies that there may be a role for the use of combination therapy; this represents a significant shift in the clinical practice guidelines. Further bolstering this trend are the recent findings that the Thr92AlaD2 polymorphism may reduce thyroid hormone signaling, resulting in localized and systemic hypothyroidism. This strengthens the hypothesis that treatment options could be personalized, taking into consideration genotypes and comorbidities. The development of long-acting formulations of liothyronine and continued advancements in development of thyroid regenerative therapy, may propel the field closer to adoption of a physiologic thyroid hormone replacement regimen with combination therapy.

Keywords: desiccated; history; levothyroxine; liothyronine; thyroid.

Figure 1

Historical Trends in the Treatment of Hypothyroidism. In the late 1800's–mid 1900's, combination therapy via thyroid transplant, thyroid feeding, thyroid extracts, thyroglobulin, or desiccated thyroid was preferred. Treatment was monitored by clinical response, basal metabolic rate (BMR), and/or serum protein-bound iodine (PBI). Thyrotoxic symptoms were prevalent in early clinical trials. In juxtaposition, levothyroxine (LT4) monotherapy to achieve normal serum TSH levels was adopted as the standard of care in the 1970's. It has become more recognized that patients on this regimen can have residual signs and symptoms of hypothyroidism. Thus, underscoring the need for new, physiologic thyroid replacement regimens with the goal to restore thyroid hormone signaling within all tissues. With the recognition that genetic polymorphisms may play a significant clinical role, personalized medicine will likely be integrated into future clinical trials and treatment regimens.