The invention includes a nucleic acid having a sequence at least 98% homologous to SEQ.ID.NO: 1, which encodes the a subunit of canine TSH. The invention also includes a nucleic acid having a sequence at least 98% homologous to SEQ.ID.NO: 2, which encodes the .beta. subunit of canine TSH. The invention also includes a method of producing an rcTSH subunit by expressing a nucleic acid having a sequence of SEQ.ID.NO: 1 and a nucleic acid having a sequence of SEQ.ID.NO: 2 in a transgenic insect cell modified to sialylate proteins and producing a sialylated rcTSH subunit. The insect cell may be a lepidopteran cell. The rcTSH may be used for diagnosis and treatment. It may be used to diagnose canine hypothyroidism.