Wednesday, March 23, 2016
Hypothyroidism requires appropriate balancing of T4 and T3
L-Thyroxine monotherapy for athyreotic rats results in a high T4:T3 ratio at doses sufficient to normalize serum TSH levels (8). Yet, the brain, liver, and skeletal muscle tissues of these l-thyroxine–treated animals continue to exhibit markers of hypothyroidism (9), probably because of the inability of l-thyroxine monotherapy to restore tissue levels of T3 (8). This is probably a direct consequence of lower serum T3 levels and the relatively high T4 concentration in these tissues, which inactivates the type 2 iodothyronine deiodinase (D2). In the hypothalamus, loss of D2 is minimal in the presence of T4, which increases sensitivity to T4 levels and explains TSH normalization, despite relatively lower levels of serum T3. Only combination therapy with l-thyroxine plus l-triiodothyronine normalized all thyroid hormone–dependent measures (9), including serum and tissue T3 levels (8). Whether tissue-specific markers of hypothyroidism are restored with l-thyroxine monotherapy in humans remains to be determined, as does the ability of l-thyroxine plus l-triiodothyronine combination therapy to normalize the serum T4:T3 ratio without adverse events. The development of a novel drug delivery system for l-triiodothyronine would facilitate these studies (5).
In humans, a factor associated with response to combination therapy in a large clinical trial is the Thr92Ala polymorphism in the type 2 deiodinase gene (DIO2), wherein the subpopulation of patients with this genetic alteration had improved well-being and preference for combination therapy (7). This has led investigators to consider whether this polymorphism could confer a defect in the D2 pathway, but normal Thr92AlaD2 enzyme kinetics have been demonstrated (73). Only recently has the Thr92AlaD2 protein been found to have a longer half-life, ectopically localize in the Golgi apparatus, and significantly alter the genetic fingerprint in cultured cells and in the temporal pole of the human brain without evidence of reduced thyroid hormone signaling (74). The significance of these studies transcends the thyroid field—this polymorphism has now been associated with a constellation of diseases, including mental retardation, bipolar disorder, and low IQ (75). If hypothyroid carriers of Thr92AlaD2 benefit from alternate therapeutic strategies in replicate studies, then personalized medicine—based on genotype—may have a role.
Ann Intern Med. 2016 Jan 5;164(1):50-6. doi: 10.7326/M15-1799.