Background and aim: Hypothyroidism is a common clinical condition resulting from insufficient hormone levels. If left untreated, it can lead to serious complications in various organs of the body. Given its prevalence, therapeutic challenges, and the impact of clinical manifestation on patients’ quality of life, a comperehensive and up-to-date review of the scientific literature is essential; Such a review can contribute to improved clinical decision-making.
Methods: In this narrative review, studies published up to the end of 2024 were identified through keyword searches including Hypothyroidism, Diagnosis, Treatment, Levothyroxine, and Liothyronin in databases such as Google Scholar, PubMed, and Web of Science. Clinical studies, review articles, original research, and clinical guidelines focusing on the etiology, diagnosis, and treatment of hypothyroidism were included. Duplicate publications and articles outside the scope of the topic were excluded. Ultimately, 105 articles with the most relevance to the study objectives were selected for review.
Results and Conclusion: Hypothyroidisim is classified as primary, central, or peripheral based on the site of dysfunction. Biochemical diagnosis of primary hypothyroidisim is based on persistent elevation of serum thyroid-stimulating hormone (TSH) along with low free thyroxine (fT4) levels (Overt hypothyroidism), or normal fT4 levels (subclinical hypothyroidism). The full replacement dose of levothyroxine (LT4) in adults is 1.6 µg/kg/day. Dosage of LT4 is adjusted by measuring serum TSH levels 4-8 weeks after the initiation of treatment. Monotherapy with liothyronine (LT3) is not recommended due to its short serum half-life and significant fluctuations. After achieving normal levels of TSH and fT4, a significant proportion of patients treated with LT4 continue to experience persistent symptoms that adversely affect their quality of life. Elevated serum T4 levels and low T3 levels may be responsible for the persistence of hypothyroid symptoms in some patients. Consequently, combination therapy with LT4 and liothyronine has been suggested; however, conclusive results from clinical trials are still lacking.