Interpreting thyroid function tests

Thyroid function tests in various conditions

Condition	TSH	fT4	fT3
Primary hyperthyroidism	undetectable
T3 Toxicosis	undetectable	normal
Subclinical hyperthyroidism	undetectable	normal	normal
Secondary hyperthyroidism (rare)	or normal
Primary hypothyroidism			or normal
Subclinical hypothyroidism		normal	normal
Secondary (pituitary) hypothyroidism	or normal		or normal
Sick euthyroid syndrome	or normal	or normal	or normal

Atypical thyroid function test results

Test	Possible cause
Suppressed TSH and normal T4 and T3	Subclinical hyperthyroidism recovery from hyperthyroidism excess thyroxine replacement Sick euthyroidism euthyroid multinodular goitre
Detectable TSH and elevated T4	Amiodarone TSH secreting pituitary tumour (rare) Thyroid hormone resistance Heterophile antibodies.

Amiodarone and thyroid function

	TSH	Free T4	Free T3
Amiodarone 'effect' (no treatment required)	Raised in first 3 months then normal	Increased	Normal or slightly decreased
Thyrotoxicosis	Undetectable	Increased	Normal or increased
Hypothyroidism	Raised	Normal or low	Low

NB Check thyroid function (TSH and fT3) in all patients prior to starting Amiodarone therapy and every 6 months thereafter.

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Hyperthyroidism

Clinical Features

Symptoms	Signs
Weight loss (weight gain in 10% of patients), increased appetite	Warm moist palms, onycholysis.
Heat intolerance, increased sweating	Tremor
Tremor, fatigue, weakness	Tachycardia, atrial fibrillation
Palpitations, breathlessness	Hyper-reflexia, proximal myopathy
Anxiety, insomnia, altered mood	Goitre
Weakness on climbing stairs or getting out of a chair	Lid-lag and lid retraction
Increased stool frequency	Thyroid eye disease: exophthalmos, periorbital oedema, ophthalmoplegia
Pruritus, hair loss	Congestive cardiac failure.
Oligomenorrhoea/amenorrhoea

Investigations

1. TSH, Free T4	If fT4 normal and TSH suppressed, then request fT3
2. Thyroid peroxidase antibodies
3. FBC	Mild neutropenia is a feature of thyrotoxicosis and is not a contraindication to antithyroid drug treatment. However neutropenia developing following the initiation of antithyroid drugs is an indication to discontinue antithyroid drugs and should be discussed with an Endocrinologist
4. ESR	If subacute thyroiditis (see link) is suspected e.g. throat pain, preceding viral illness.
5. Thyroid uptake scan	Only if the diagnosis of Graves' disease is unclear e.g. negative thyroid peroxidase antibodies and no evidence of thyroid eye disease

Management

Once the diagnosis is confirmed, start Carbimazole therapy, 20-40mg OD depending on concentration of fT4, and refer to Endocrine clinic. Start Propranolol (80mg BD/TDS) if severely symptomatic with no contraindications.

Monitoring

TSH and fT4 every 4-8 weeks. Once fT4 is in the normal range, reduce carbimazole
N.B. TSH may remain suppressed for a few weeks after normalisation of fT4. As agranulocytosis occurs very suddenly, routine monitoring of full blood count is of little use. Patients typically present with fever and evidence of infection, usually in the oropharynx, and each patient should therefore receive written instructions to discontinue the medication and contact their doctor for a blood count should the situation arise.

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Subacute thyroiditis

Clinical features

Malaise, pain and tenderness in throat/neck, in addition to symptoms & signs of thyrotoxicosis. May alternatively present as thyrotoxicosis following a viral infection, usually URTI.

Investigations

Hyperthyroid thyroid function in addition to raised ESR. Thyroid peroxidase antibodies may be positive.

Treatment

Symptomatic - nonsteroidal anti-inflammatory drugs and propranolol.
There is no place for antithyroid drugs.
Hypothyroid phase may follow and requires thyroxine therapy.

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Hyperthyroidism and Pregnancy

Pre-existing hyperthyroidism

Change from carbimazole to propylthiouracil (Dose equivalent - Carbimazole 5mg=Propylthiouracil 50mg) and inform endocrine clinic.

New onset thyrotoxicosis

Start propylthiouracil (200-400mg/day) and refer urgently to Endocrine clinic

Postpartum thyroiditis

7% of pregnancies, especially in patients with other autoimmune disease e.g. type 1 DM, and in those with positive thyroid peroxidase antibodies.
Transient hyper or hypothyroidism, with most women becoming euthyroid by 12 months postpartum.
NB High recurrence rate in subsequent pregnancies.

The hyperthyroid phase may occur any time between 2-10 months postpartum. The most common symptom is fatigue, but thyrotoxic symptoms may also occur. Treatment of postpartum thyroiditis is β -blockers if symptomatic. There is no place for antithyroid drugs. -blockers can be stopped once TFTs have normalised, typically within 3 months. Hypothyroid phase occurs 2-12 months postpartum. Treat all symptomatic patients and all with TSH>10. Treatment is continued for 6-12 months and then attempt to wean off T4.

NB 25-30% develop permanent hypothyroidism and thus all women who develop postpartum thyroiditis should be followed up with annual TSH.

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Subclinical Hyperthyroidism

Diagnosis

Suppressed TSH but normal fT4 and fT3

Management

6 monthly TSH and fT3

Indications for referral to Endocrinologist

Female seeking pregnancy
Presence of AF or cardiovascular disease
Possible thyrotoxic symptoms
Enlarging goitre
Osteoporosis
Development of overt thyrotoxicosis

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Hypothyroidism

The most common cause is autoimmune, e.g. Hashimoto's disease

Clinical symptoms and signs

Insidious non-specific onset
Fatigue, lethargy
Decreasing appetite and weight gain
Constipation
Cold intolerance
Bradycardia
Cramps
Menorrhagia
Slowing of intellectual and motor activities
Dry skin; hair loss
Deep hoarse voice

Diagnosis

TSH raised fT4 low

Treatment

Thyroxine 100mcg OD. If elderly, H/O IHD or severe hypothyroidism then start at 25mcg OD and gradually increase dose by 25mcg every 4 weeks.
Monitor TFTs every 6 weeks until euthyroid (i.e. TSH <2mU/L), then 6-12 monthly thereafter

Subclinical Hypothyroidism

TSH raised fT4 normal

When to treat subclinical hypothyroidism?

Thyroid peroxidase antibody positive
TSH >10mU/L
Symptomatic
Seeking fertility or pregnant - Refer to Endocrine clinic
Previous treatment for thyrotoxicosis e.g. radioiodine therapy

Hypothyroidism in Pregnancy

Refer to Endocrinologist.
Note, thyroxine requirements increase during pregnancy so TFTs should be checked:
Aim: TSH <2 and normal fT4
Reduce thyroxine to pre-pregnancy dose after delivery and recheck TFTs 6 weeks postpartum

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Multinodular Goitre and Solitary Adenomas

Indications for referral

New onset solitary nodule in a male or >65 yrs
Enlarging goitre/nodule
Pressure symptoms eg stridor, dysphagia
Presence of: hoarse voice, cervical lymphadenopathy - URGENT referral required
Hyperthyroidism

Investigation prior to referral

TSH, fT4