On average, the normal thyroid releases about 100 mcg of thyroxine (T4) daily and only small amounts of triiodothyronine (T3). Levothyroxine (either natural or synthetic T4), a prohormone, is converted to liothyronine (natural or synthetic T3), the active hormone in the peripheral tissues. Decreased production of T4 causes an increase in secretion of TSH by the pituitary. TSH causes the thyroid to release more T3 by stimulating thyroid T4-5'-deiodinase activity and stimulates hyperplasia and hypertrophy of the thyroid. Decreasing levels of T4 and increasing thyroid production of T3 leads to preservation of T3 levels and lowering of T4 levels early in the disease.
Hypothyroidism is divided into primary hypothyroidism (failure of the thyroid gland to produce hormones); secondary hypothyroidism (the thyroid gland is normal and the pituitary fails to secrete adequate thyrotropin [TSH]); and tertiary hypothyroidism (failure to secrete thyrotropin releasing-hormone [TRH]).
There are several distinct causes for chronic hypothyroidism, most common being Hashimoto's thyroiditis and postoperative or hypothyroidism following radioiodine therapy for hyperthyroidism.
Thyroid hormone is very important to neural development in the neonatal period. A deficiency of thyroid hormones can lead to cretinism. For this reason it is important to detect and treat thyroid deficiency early. In Australia and many other countries this is done by testing for TSH on the routine neonatal heel pricks performed by law on all new born babies.
- Hashimoto's thyroiditis information
Reduction or loss of TSH secretion by the pituitary is a very rare cause of hypothyroidism.
(or Doctor induced).
Cretinism refers to congenital hypothyroidism, and myxedema coma refers to the most severe form of hypothyroidism.
Symptoms and signs
Severity of hypothyroidism is variable. Some patients are classified as subclinical hypothyroid when only diagnostic findings show thyroid hormone abnormality. Others have moderate symptoms that can be mistaken for other diseases and states. Advanced hypothyroidism is easily recognized even for non-specialist.
Hypothyroidism signs and symptoms are :
- Slowed speech and impaired memory
- Cold intolerance
- A slow heart rate and sluggish reflexes
- Dry puffy skin and hair loss
- Can contribute to depression
- May lead to weight gain
In the elderly, hypothyroidism is relatively common and can be easily missed. It is an important but treatable cause of depression in this age group.
Elevated TSH and decreased T3 and T4 levels.
Substitution of thyroid hormones by taking thyroxine (T4) tablets. Doses are started with smaller amounts of thyroxine and then slowly titrated under control of TSH levels. Usually the maintenance dose is about 1 to 2 micrograms (ug) per kilogram of body weight. Controversy exists about the usefulness of administrating triiodothyronine (T3) as well as T4. Some thyroid deficiency is a side effect of treatment for thyroid cancer, and Graves' disease. Deficiencies of some dietary minerals can lead to hypothyroidism. Supplementation can be an effective treatment.
Follow up with labs: Patients with stable thyroid status and intact hypothalamic-pituitary function, serum thyroid stimulating hormone (TSH) measurement is more sensitive than free thyroxine (FT4) for detecting mild (subclinical) thyroid hormone excess or deficiency.
In patients with unstable thyroid status, serum FT4 measurement is a more reliable indicator of thyroid status than TSH when thyroid status is unstable, such as during the first 2-3 months of treatment for hypo- or hyperthyroidism. In hypothyroid patients suspected of intermittent or non-compliance with L-T4 replacement therapy, both TSH and FT4 should be used for monitoring. Non-compliant patients may exhibit discordant serum TSH and FT4 values (high TSH/ high FT4) because of persistent disequilibrium between FT4 and TSH.