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Back to Endocrine Diseases
Hypothyroidism
(Myxedema)
Hypothyroidism is a pathologic state caused by insufficient secretion
of thyroid hormones by the thyroid gland.
Pathophysiology
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.
Causes
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.
Neonatal period
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
Pituitary failure
Reduction or loss of TSH secretion by the pituitary is a very rare cause
of hypothyroidism.
Iatrogenic
(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.
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Diagnosis
Elevated TSH and decreased
T3 and
T4 levels.
Treatment
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
(?g) 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.
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