Treatment of hypothyroidism: all that glitters is gold?

Abstract

Levothyroxine (L-T4) is recommended worldwide for replacement therapy of hypothyroidism due to its efficacy in resolving the symptoms, long-term experience of its benefits, long serum half-life, and low cost. Since the isolation of L-T4 by Kendall in 1914 [1] and then the synthesis of T4 and its better absorbed sodium salt [2], it has become the therapy of choice, also due to the demonstration that about 80 % of biologically active tri-iodiothyronine is derived from peripheral conversion of T4 [3].The individually tailored dose of L-T4 therapy depends on different factors such as body weight, lean body mass, pregnancy status, degree of thyrotropin (TSH) elevation, age, comorbidities, etiology of hypothyroidism, and consequently amount of residual functional thyroid tissue. Therefore, thyroid hormone need may considerably vary among patients. As a matter of fact, although therapy is given correctly, over-as well as under-treatment may quite frequently be observed in clinical practice with relevant impact on different clinical outcomes. Indeed, complications of under-treatment are detrimental effects on the serum lipid profile [4], progression of atherosclerotic cardiovascular disease [5], and congestive heart failure [6].Nowadays, evidence-based guidelines recommend 1 h interval between L-T4 tablet ingestion and breakfast to achieve the best therapeutic effects [7]. The absorption of an orally daily administered dose of L-T4 is about 70-80 % under optimal fasting conditions [8]. Nevertheless, almost 50 % of treated patients show persistently abnormal thyroid functional tests 1 year after starting treatment [9]. This may be due, at least in part, as for many other chronic treatments [10], to compliance problems. Specifically, a subset of patients find difficult or even unfeasible, due for example to work requirements, to comply with the indication of assuming the tablet 1 h before breakfast. This often leads to disproportioned increase in doses of L-T4 and improper or expensive work-up [11].Moreover, different pathologic conditions, such as atrophic and Helicobacter Pylori-related gastritis or celiac disease, could reduce the rate of absorption of L-T4 via modification of the gastric acid pH, that is essential for complete dissolution of the tablet [12,13].Furthermore, soy products and several drugs, such as calcium carbonate, bile acid sequestrants, proton pump inhibitors, and ferrous sulfate, may alter L-T4 absorption [7].Trying to improve these pharmacokinetic drawbacks, recently new formulations of L-T4 have been introduced: soft gel capsules, containing T4 dissolved in glycerin, and the liquid formulation (only in a few countries). These preparations have been reported to reach the systemic circulation more quickly since gastric dissolution is not needed [14]. In a recent two-year prospective study involving hypothyroid patients without malabsorption or drug interference, Fallahi et al. reported higher effectiveness of liquid L-T4 formulation in normalizing TSH levels as compared to L-T4 tablets [15]. Indeed...

Expert opinion on liquid L-thyroxine usage in hypothyroid patients and new liquid thyroxine formulation — Tirosint SOL [Opinia ekspertów dotycząca stosowania płynnej postaci lewotyroksyny oraz nowego preparatu Tirosint SOL u chorych na niedoczynność tarczycy]

“…The faster absorption of liquid LT4 to systemic circulation might be due to omitting the stomach phase of disintegration and dissolution. Also, partial oral absorption by well-vascularised oral mucosa of liquid L-T4 in alcoholic solution is suggested by some authors [23,24].…”

Levothyroxine Sodium Pentahydrate Tablets – Formulation Considerations

“…The dose of levothyroxine sodium for each patient has to be "tailored and titrated" and depends on numerous factors including patient's age, disease state, cardiovascular status, other comorbidities, lean body mass, pregnancy status, coadministered medications and severity of hypothyroidism (TSH and T4 levels). [140][141][142][143][144] Levothyroxine is absorbed predominantly through the jejunal-ileal section of the gastro-intestinal tract. The absorption is highly variable and incomplete, hence limiting the bioavailability to 40−80%.…”

Drug interactions in users of tablet vs. oral liquid levothyroxine formulations: a real-world evidence study in primary care

“…In addition, with the reduced rates of TSH variability and need for daily dosage increase, patients are actually more protected against the risk of exposure to excessive dose of replacement therapy, whenever the interacting drugs are discontinued. This is of particular relevance in patients with or at risk of coronary artery disease, arrhythmias, and bone fractures [53,54]. Therefore, since optimization of LT4 replacement therapy plays a key role in the successful management of hypothyroidism, in the scenario of diverse LT4 formulations availability, clinicians should carefully consider these findings to tailor patients therapy whenever co-prescription of LT4 and potentially interacting drugs is needed.…”