last authored: Oct 2009, David LaPierre
Hypothyroidism, measured by low free T4, develops over months or years.
a simple case introducing clincial presentation and calling for a differential diagnosis. To get students thinking.
It can be caused by
thyroiditis (autoimmune or Hashimoto's)
secondary hypothyroidism can be caused by various manifestations of hypopituitarism.
iodide deficiency, or congenital causes.
Affects 1:4000 births, with a female:male ration of 2:1. It is usually caused by thyroid malformation, though maternal factors such as iodine deficiency, antithyroid medications, or gastrogen can also cause it.
Pathologically increased levels of TRH cause an increase in prolactin, though it is unknown if there is any physiologic relevance of this. Increased prolactin then inhibits GnRH secretion via negative feedback, blocking LH and FSH production and thereby the menstrual cycle.
Hashimoto's: many people have antibodies against thyroglobulin, thyroid peroxidase,
Hypothyroid signs and symptoms revolve around a decreased metabolic rate.
Symptoms of hypothyroidism include
Congenital hypothyroidism is usually asymptomatic due to maternal T4, but can cause:
cardiovascular: decreased cardiac output
head and neck
Congenital hypothyroidism can cause:
Newborns are often routinely screened for TSH or T4.
Tissue markers of hypothyroidism
Uptake and scans are not helpful, as all images will show low uptake.
Synthroid is used to replace T4. Target TSH between 1-2.
Is secondary hypothyroidism, target T3 and T4 levels to their normal range.
In subacute thyroiditis, leave people on synthroid for 6-8 weeks, then gradually withdraw to see what happens.
Take on an empty stomach, especially avoiding cations.
Aim for a dose of 1.6 ug/kg, and adjust every 4-6 weeks, as T4's half-life is long.
If heart disease, start at 25 ug daily and increase by 25 ug every 2 weeks until at target dose to avoid sympathomemetic effects.
If adrenal insufficiency, need stress doses of corticosteroids.
As the negative feedback on the pituitary is lost, TSH levels increase in an effort to restore T3/T4 levels. Following the commencement of treatment, T4 replacement, TSH levels return.
Congenital hypothyroidism has an excellent prognosis if treatment with thyroxine is started within 1-2 months. However, if treatment is delayed 3-6 months, permanent developmental delay or mental retardation (cretinism) can result.
Case #2 - a small story wrapping it all up and asking about esp management.
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