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Hypothyroidism: A Diagnostic Approach

Hypothyroidism: A diagnostic approach A 17-year-old girl is brought to your office
complaining of progressive weight gain of 4 kg or 8 pounds in 6 months, fatigue, and
cold intolerance. Her pulse is 55 beats per minute. Other vital signs are normal. Examination
shows dry skin and delayed relaxation of the ankle jerk reflex, also termed the Woltman
sign. With these clinical signs of hypothyroidism in mind, thyroid function tests are requested. Laboratory studies reveal increased TSH levels
with 12 milli-international units per litre and decreased FT4 levels with 0.3 micrograms
per deciliter. Let’s take a close look at how to interpret
thyroid function tests. The first marker used to assess thyroid function is TSH, thyroid
stimulating hormone. If the TSH level is normal, hypothyroidism is unlikely. On the other hand,
a high TSH indicates primary hypothyroidism, whereas a low TSH indicates secondary or tertiary
hypothyroidism. The next value to look at is FT4, free thyroxine.
A high TSH and normal FT4 indicates subclinical hypothyroidism, a condition in which the thyroid
gland recognises thyroid hormone deficiency, resulting in an increased TSH; however, FT4
levels have not yet decreased. On the other hand, decreased FT4 levels suggests a relevant
deficiency in thyroid hormones and is classified as primary hypothyroidism. Let’s go back to the interpretation of TSH
values in the first level of the flowchart. A low TSH level together with a low FT4 would
suggest secondary or tertiary hypothyroidism. Secondary hypothyroidism is due to the failure
of the pituitary gland to secrete TSH, whereas tertiary hypothyroidism is caused by the failure
of the hypothalamus to release TRH, thyrotropin-releasing hormone. In both cases, the thyroid gland
does not produce enough FT4 causing hypothyroidism. To confirm the exact cause, further follow
up is required. Ok, let’s analyze the patient’s laboratory
findings. The reference value for TSH is 0.5 to 5 mIU/L. An elevated TSH value of 12 was
found. The reference value for FT4 in adults is 0.7 to 1.8 micrograms per deciliter. A
low FT4 of 0.3 was detected in the patient. Based on these laboratory findings, the patient
has primary hypothyroidism. Information regarding further follow up and management can be found
in the respective chapter.

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