Hypothyroidism
last authored: Oct 2009, David LaPierre
Introduction
Hypothyroidism, measured by low free T4, develops over months or years.
The Case of...
a simple case introducing clincial presentation and calling for a differential diagnosis. To get students thinking.
Causes and Risk Factors
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.
- Hashimoto's and other autoimmune
- iatrogenic hypothyroidism is common following treatment of hyperthyroidism.
- iodine deficiency
- exhaustive phase of subacute thyroiditis
- drugs
- secondary or tertiary disease from pituitary or hypothalamic dysfunction
Congenital hypothyroidism
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.
Pathophysiology
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,
- potentially a loss of T suppressor cells
- can have CD4 activation
- plasma cell production
Signs and Symptoms
Hypothyroid signs and symptoms revolve around a decreased metabolic rate.
- history
- physical exam
History
Symptoms of hypothyroidism include
- weight gain
- cold sensitivity
- lethargy and mental slowness
- depression
- amenorrhea
- psychosis (myxedema madness)
Congenital hypothyroidism is usually asymptomatic due to maternal T4, but can cause:
- constipation
- lethargy
- poor feeding
- sluggish, hoarse cry
Physical Exam
cardiovascular: decreased cardiac output
- bradycardia
- hypotension
head and neck
- droopy eyelids, periorbital edema
- loss of lateral eyebrow
- goiter
- dry, coarse, thickened skin
- myxedema
- growth and mental retardation
- delayed reflex relaxation (CMAJ video demonstration)
- effusions: pleural, pericardial
Congenital hypothyroidism can cause:
- macroglossia
- coarse facial features
- large fontanelles
- umbilical hernia
Investigations
- lab investigations
- diagnostic imaging
Lab Investigations
Newborns are often routinely screened for TSH or T4.
Tissue markers of hypothyroidism
- elevated CK
- hyponatremia
- macrocytic anemia
- hypercholesterolemia due to T3's positive effect on LDL binding to its receptor and consequent uptake
Diagnostic Imaging
Uptake and scans are not helpful, as all images will show low uptake.
Differential Diagnosis
Treatments
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.
Consequences and Course
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.
The Case of...
Case #2 - a small story wrapping it all up and asking about esp management.
Additional Resources
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