1. Congenital Hypothyroidism
Congenital hypothyroidism is characterized by a deficiency of thyroid hormone in newborns. It is a fatal condition that requires prompt diagnosis to prevent irreversible neurological damage. Research suggests that immediate treatment of levothyroxine (L-T4) upon diagnosis, preferably within the first two weeks after birth, successfully reduces the risk of intellectual deficits and optimizes neurodevelopment [2].
Furthermore, T4 is considered a preferred treatment for congenital hypothyroidism [2]. One study concluded that combination treatment with T4 plus T3 didn’t show any significant advantage over T4 therapy in patients with congenital hypothyroidism [4].
The effective treatment of congenital hypothyroidism typically involves a single morning dose of oral levothyroxine[3]. However, an initial study in adult patients suggested that levothyroxine administration in the evening might yield at least an equal, if not better, therapeutic effect compared to morning administration [5].
2. Polycystic ovary syndrome (PCOS)
According to the guidelines of the American Thyroid Association, hypothyroid women aiming for pregnancy should modify their levothyroxine dosage to attain a TSH level within the range of the lower reference limit and 2.5 mIU/L. However, there is insufficient evidence to establish whether levothyroxine supplementation enhances fertility in anti-TPO antibody-positive women with subclinical hypothyroidism who are trying to conceive naturally [6].
In a small case-control study, levothyroxine treatment in women with polycystic ovary syndrome (PCOS) and subclinical hypothyroidism (SCH) demonstrated no significant impact based on multivariate analysis. The T4 treatment did not alter serum metabolic markers, including LDL and HDL cholesterol, triglycerides, glucose, and insulin [7].
3. Multiple Sclerosis
Multiple sclerosis (MS) is an autoimmune disease characterized by the demyelination of nerves. Stats show that approximately 70% of individuals with MS experience impaired and demyelinated visual systems. Oligodendrocyte progenitor cells (OPC) play an important role in the generation of oligodendrocytes, which are responsible for the myelination of nerve fibers. Research shows that the effectiveness of this process depends on various factors including thyroid hormones [8].
According to one study in male Wistar rats’, intraperitoneal administration of levothyroxine at doses of 50 and 100 μg/kg effectively prevented impairments in visual evoked potentials induced by lysolecithin. However, the lower levothyroxine dose (20 μg/kg) did not exhibit any effects [9]. These findings suggest that higher doses of thyroid hormones have more favorable effects on the healing process following demyelination induced by lysolecithin injection.
4. Nonalcoholic Fatty Liver Disease (NAFLD)
Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of extrahepatic fat in the absence of excessive alcohol consumption. The rising prevalence of NAFLD is contributing to a growing need for liver transplantation. As levothyroxine has been shown to reduce body weight and serum lipid levels, it might alleviate the negative side effects of NAFLD[10].
One study found that levothyroxine replacement therapy had a positive impact on NAFLD patients with subclinical hypothyroidism (SCH). It reduced the prevalence of NAFLD and lowered hepatic enzyme levels in serum. The results of the study suggest that thyroxine supplementation could be an effective treatment to manage NAFLD in patients with SCH [11].
