side effects of too much Levothyroxine, and of too little?

Hello and welcome to e health forum.

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are tyrosine-based hormones ‎produced by the thyroid gland primarily responsible for regulation of metabolism.‎

These hormones act on nearly every cell in the body. They act to increase the basal metabolic rate, ‎affect protein synthesis, help regulate long bone growth (along with growth hormone). These hormones also regulate protein, fat, and carbohydrate metabolism, ‎affecting how human cells use energetic compounds. They also stimulate vitamin metabolism.‎

After surgery for thyroid cancer, thyroid hormone (levothyroxine) is needed both to replace the function of the removed thyroid gland and to keep any small or residual amounts of thyroid cancer cells from growing. When thyroid hormone is used to treat hypothyroidism, the goal of treatment is to keep thyroid function within the normal range.

Early symptoms due to hypothyroidism present as – cold sensitivity, Constipation, low mood or ‎Depression, Fatigue or feeling slowed down, Heavier menstrual periods, Joint or muscle pain, ‎Paleness or dry skin, Thin, brittle hair or fingernails, Weakness , Weight gain (unintentional)‎. If undetected and untreated further complications like female infertility, elevated serum cholesterol, bradycardia, hoarseness of voice, anemia, hair loss, impaired cognitive function and ocular changes develop.

Hence in order to prevent such conditions daily long term ( in some cases life long) thyroid hormone supplementation is needed. In addition it is important to monitor the effect of treatment both clinically as well as with regular testing of thyroid function tests (a simple blood test). Regular monitoring will help in titration of the dose, which may be increased or decreased as needed. Such titration prevents excess thyroid hormones being given, hence preventing hyperthyroidism.

The major form of thyroid hormone in the blood is thyroxine (T4), which is the storage form of the hormone with a longer half-life than T3. Thyroxine is converted to T3, the active form of the hormone, which is 3-4 times more potent than T4. The ratio of T4 to T3 released into the blood is roughly 20 to 1.

Taking a Combination T4/T3 preparations contain much more T3 than is usually produced naturally within the body. It is also given once a day, ignoring the short life span of T3 in the body. Because of this, immediate absorption of T3 in the preparation, patients can have side effects (symptoms of hyperthyroidism - such as rapid heart beat, insomnia and anxiety).

I hope this answers your questions.

You may consult with your treating physician for further guidance and proper treatment.