There are no official guidelines to assist in answering this question. I will search for relevant research papers instead.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Efficacy of Sublingual Levothyroxine (T4) Therapy
There is limited information available on the efficacy of sublingual levothyroxine (T4) therapy. However, the available evidence on levothyroxine (T4) therapy can be summarized as follows:
- Levothyroxine (LT4) is effective for most patients with hypothyroidism, but a minority of patients remain symptomatic despite normalization of serum thyrotropin levels 1.
- New formulations of LT4, such as soft gel and liquid formulations, have been developed to improve bioavailability and facilitate absorption 2, 3.
- These new formulations may be more suitable for patients with variations in bioavailability, such as those with gastrointestinal disorders or taking interfering medications 2, 3.
- However, the evidence in favor of using these new formulations over traditional LT4 tablets is weak, and additional studies are needed to investigate their potential benefits 2.
- Factors that affect the gastrointestinal absorption of LT4 include gastrointestinal disorders, certain foods and beverages, and concomitant medications 4.
- LT4/LT3 combination therapy may be an option for patients with persistent symptoms of hypothyroidism despite adequate control of TSH levels, but the evidence is limited and more studies are needed 5.
Key Findings
- Levothyroxine (LT4) is effective for most patients with hypothyroidism, but some patients may require alternative therapies 1, 5.
- New formulations of LT4, such as soft gel and liquid formulations, may improve bioavailability and facilitate absorption 2, 3.
- Gastrointestinal disorders, certain foods and beverages, and concomitant medications can affect the absorption of LT4 4.
- LT4/LT3 combination therapy may be an option for patients with persistent symptoms of hypothyroidism, but more studies are needed to confirm its efficacy 5.