Hypothyroidism — an underactive thyroid gland that produces insufficient thyroid hormone — is one of the most common endocrine disorders, affecting an estimated 5% of people aged 12 and over in the United States, with higher prevalence in women and the elderly. The thyroid gland produces two hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the primary circulating form and is peripherally converted to the more active T3 by deiodinase enzymes in target tissues. In primary hypothyroidism, the thyroid fails to produce enough T4/T3 — resulting in an elevated TSH (thyroid-stimulating hormone, which rises as the pituitary tries to stimulate the gland) and symptoms including fatigue, cold intolerance, weight gain, constipation, dry skin, hair loss, depression, and slowed cognition.
Treatment for hypothyroidism involves replacing the missing hormone with synthetic levothyroxine (T4), which is then peripherally converted to T3 as needed. Levothyroxine is one of the most prescribed medications in the United States. It is a narrow therapeutic index drug — meaning the dose must be calibrated precisely for each patient using TSH monitoring, and even small formulation differences between brands or generics can affect thyroid function. Patients are typically advised to take the same brand or generic manufacturer consistently and to not switch without close monitoring.
Thyroid Medications Available at Lucas Clinic
- Synthroid (Levothyroxine) — synthetic T4; first-line treatment for hypothyroidism; 12 dose strengths from 25–300 mcg; colour-coded tablets; take fasting 30–60 min before breakfast
Disclaimer: This page is for general informational purposes only and does not constitute medical advice. Always consult a licensed healthcare professional before making any treatment decisions. See our full disclaimer.